doc: Nuke os_unix.txt

All of this information is a combination of incorrect, outdated, or
redundant given its availability in other help files.

Reviewed-by: Justin M. Keyes <justinkz@gmail.com>
Reviewed-by: Felipe Morales <hel.sheep@gmail.com>

runtime/doc/eval.txt

@@ -6888,7 +6888,6 @@ fname_case		Case in file names matters (for MS-DOS and Windows
 			this is not present).
 folding			Compiled with |folding| support.
 footer			Compiled with GUI footer support. |gui-footer|
-fork			Compiled to use fork()/exec() instead of system().
 gettext			Compiled with message translation |multi-lang|
 gui			Compiled with GUI enabled.
 gui_athena		Compiled with Athena GUI.
@@ -6947,7 +6946,6 @@ statusline		Compiled with support for 'statusline', 'rulerformat'
 syntax			Compiled with syntax highlighting support |syntax|.
 syntax_items		There are active syntax highlighting items for the
 			current buffer.
-system			Compiled to use system() instead of fork()/exec().
 tag_binary		Compiled with binary searching in tags files
 			|tag-binary-search|.
 tag_old_static		Compiled with support for old static tags

runtime/doc/help.txt

@@ -161,7 +161,6 @@ Versions ~
 						*sys-file-list*
 Remarks about specific systems ~
 |os_mac.txt|	Macintosh
-|os_unix.txt|	Unix
 |os_win32.txt|	MS-Windows
 						*standard-plugin-list*
 Standard plugins ~

runtime/doc/os_unix.txt

@@ -1,57 +0,0 @@
-*os_unix.txt*   For Vim version 7.4.  Last change: 2005 Mar 29
-
-
-		  VIM REFERENCE MANUAL    by Bram Moolenaar
-
-
-							*unix* *Unix*
-This file contains the particularities for the Unix version of Vim.
-
-For compiling Vim on Unix see "INSTALL" and "Makefile" in the src directory.
-
-The default help file name is "/usr/local/lib/vim/help.txt" The files
-"$HOME/.vimrc" and "$HOME/.exrc" are used for configuration.  Additionally
-"/usr/local/etc/vimrc" is used first.  If "/usr/local/share" exists it is used
-instead of "/usr/local/lib".
-
-Temporary files (for filtering) are put in "/tmp".  If you want to place them
-somewhere else, set the environment variable $TMPDIR to the directory you
-prefer.
-
-With wildcard expansion you can use '~' (home directory) and '$'
-(environment variable).
-
-							*fork* *spoon*
-For executing external commands fork()/exec() is used when possible, otherwise
-system() is used, which is a bit slower.  The output of ":version" includes
-|+fork| when fork()/exec() is used, |+system()| when system() is used.  This
-can be changed at compile time.
-
-Because terminal updating under Unix is often slow (e.g. serial line
-terminal, shell window in suntools), the 'showcmd' and 'ruler' options
-are off by default.
-
-When using Vim in an xterm the mouse clicks can be used by Vim by setting
-'mouse' to "a".  If there is access to an X-server gui style copy/paste will
-be used and visual feedback will be provided while dragging with the mouse.
-If you then still want the xterm copy/paste with the mouse, press the shift
-key when using the mouse.  See |mouse-using|.
-
-							*terminal-colors*
-To use colors in Vim you can use the following example (if your terminal
-supports colors, but "T_Co" is empty or zero): >
-   :set t_me=^[[0;1;36m     " normal mode (undoes t_mr and t_md)
-   :set t_mr=^[[0;1;33;44m  " reverse (invert) mode
-   :set t_md=^[[1;33;41m    " bold mode
-   :set t_se=^[[1;36;40m    " standout end
-   :set t_so=^[[1;32;45m    " standout mode
-   :set t_ue=^[[0;1;36m     " underline end
-   :set t_us=^[[1;32m       " underline mode start
-[the ^[ is an <Esc>, type CTRL-V <Esc> to enter it]
-
-For real color terminals the ":highlight" command can be used.
-
-The file "tools/vim132" is a shell script that can be used to put Vim in 132
-column mode on a vt100 and lookalikes.
-
- vim:tw=78:ts=8:ft=help:norl:

runtime/doc/various.txt

@@ -332,7 +332,6 @@ N  *+find_in_path*	include file searches: |[I|, |:isearch|,
 			|CTRL-W_CTRL-I|, |:checkpath|, etc.
 N  *+folding*		|folding|
    *+footer*		|gui-footer|
-   *+fork*		Unix only: |fork| shell commands
 N  *+gettext*		message translations |multi-lang|
    *+GUI_Athena*	Unix only: Athena |GUI|
    *+GUI_neXtaw*	Unix only: neXtaw |GUI|
@@ -377,7 +376,6 @@ N  *+startuptime*	|--startuptime| argument
 N  *+statusline*	Options 'statusline', 'rulerformat' and special
 			formats of 'titlestring' and 'iconstring'
 N  *+syntax*		Syntax highlighting |syntax|
-   *+system()*		Unix only: opposite of |+fork|
 N  *+tag_binary*	binary searching in tags file |tag-binary-search|
 N  *+tag_old_static*	old method for static tags |tag-old-static|
 m  *+tag_any_white*	any white space allowed in tags file |tag-any-white|