Remove all invalid nvimrc and ngvimrc references

"vimrc" refers to all files that are used to configure Neovim. The main
configuration file is init.vim nowadays.

All nvimrc references that are left refer to a local ".nvimrc" which is read
if 'exrc' is set. ".ngvimrc" references were completely wiped.

Closes #3552.

runtime/doc/gui.txt

@@ -61,10 +61,6 @@ When the GUI starts up initializations are carried out, in this order:
     ~/.config).
   The name of the first file found is stored in $MYGVIMRC, unless it was
   already set.
-- If the 'exrc' option is set (which is NOT the default) the file ./.ngvimrc
-  is sourced, if it exists and isn't the same file as the system or user
-  gvimrc file.  If this file is not owned by you, some security restrictions
-  apply.  When ".ngvimrc" is not found, "_ngvimrc" is tried too.
 
 NOTE: All but the first one are not carried out if Vim was started with
 "-u NONE" and no "-U" argument was given, or when started with "-U NONE".

runtime/doc/options.txt

@@ -2295,13 +2295,13 @@ A jump table for the options with a short description can be found at |Q_op|.
 					*'exrc'* *'ex'* *'noexrc'* *'noex'*
 'exrc' 'ex'		boolean (default off)
 			global
-	Enables the reading of .nvimrc, .exrc and .ngvimrc in the current
-	directory.  If you switch this option on you should also consider
-	setting the 'secure' option (see |initialization|).  Using a local
-	.exrc, .nvimrc or .ngvimrc is a potential security leak, use with care!
-	also see |init.vim| and |gui-init|.
+	Enables the reading of .nvimrc and .exrc in the current directory.
+	If you switch this option on you should also consider setting the
+	'secure' option (see |initialization|).  Using this option comes
+	with a potential security risk, use with care!
 	This option cannot be set from a |modeline| or in the |sandbox|, for
 	security reasons.
+	Also see |init.vim| and |gui-init|.
 
 				*'fileencoding'* *'fenc'* *E213*
 'fileencoding' 'fenc'	string (default: "")