docs: Remove RISC OS remnants

runtime/doc/filetype.txt

@@ -36,7 +36,6 @@ Detail: The ":filetype on" command will load one of these files:
 		Amiga	    $VIMRUNTIME/filetype.vim
 		Mac	    $VIMRUNTIME:filetype.vim
 		MS-DOS	    $VIMRUNTIME\filetype.vim
-		RiscOS	    Vim:Filetype
 		Unix	    $VIMRUNTIME/filetype.vim
 		VMS	    $VIMRUNTIME/filetype.vim
 	This file is a Vim script that defines autocommands for the

runtime/doc/options.txt

@@ -5688,9 +5688,6 @@ A jump table for the options with a short description can be found at |Q_op|.
 					Macintosh: "$VIM:vimfiles,
 						$VIMRUNTIME,
 						$VIM:vimfiles:after"
-					RISC-OS: "Choices:vimfiles,
-						$VIMRUNTIME,
-						Choices:vimfiles/after"
 					VMS: "sys$login:vimfiles,
 						$VIM/vimfiles,
 						$VIMRUNTIME,
@@ -7603,8 +7600,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 							 "$VIM/vimfiles/view",
 				 for Unix: "~/.vim/view",
 				 for Macintosh: "$VIM:vimfiles:view"
-				 for VMS: "sys$login:vimfiles/view"
-				 for RiscOS: "Choices:vimfiles/view")
+				 for VMS: "sys$login:vimfiles/view")
 			global
 			{not in Vi}
 			{not available when compiled without the |+mksession|

runtime/doc/starting.txt

@@ -767,7 +767,7 @@ accordingly.  Vim proceeds in this order:
      a. If vim was started as |evim| or |eview| or with the |-y| argument, the
 	script $VIMRUNTIME/evim.vim will be loaded.
 							*system-vimrc*
-     b. For Unix, MS-DOS, MS-Windows, VMS, Macintosh, RISC-OS and Amiga
+     b. For Unix, MS-DOS, MS-Windows, VMS, Macintosh, and Amiga,
 	the system vimrc file is read for initializations.  The path of this
 	file is shown with the ":version" command.  Mostly it's "$VIM/vimrc".
 	Note that this file is ALWAYS read in 'compatible' mode, since the

runtime/doc/usr_05.txt

@@ -317,7 +317,6 @@ Then copy the file to your plugin directory:
 	Amiga		s:vimfiles/plugin
 	Macintosh	$VIM:vimfiles:plugin
 	Mac OS X	~/.vim/plugin/
-	RISC-OS		Choices:vimfiles.plugin
 
 Example for Unix (assuming you didn't have a plugin directory yet): >
 

runtime/doc/vi_diff.txt

@@ -138,7 +138,6 @@ Support for different systems.
 	- Windows 95 and Windows NT, with support for long file names.
 	- VMS
 	- Macintosh
-	- Risc OS
 	- IBM OS/390
 	Note that on some systems features need to be disabled to reduce
 	resource usage, esp. on MS-DOS.  For some outdated systems you need to