runtime: Remove fallback :CompilerSet definition from compiler plugins

The :CompilerSet command was added in version Vim 6.4 which was released
twenty years ago.  Other runtime files do not support versions of that
vintage so it is reasonable to remove this fallback command definition
now.

closes: #14399

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/usr_51.txt b/runtime/doc/usr_51.txt
index f2b5e13..b0bf792 100644
--- a/runtime/doc/usr_51.txt
+++ b/runtime/doc/usr_51.txt
@@ -1,4 +1,4 @@
-*usr_51.txt*	For Vim version 9.1.  Last change: 2022 Jun 03
+*usr_51.txt*	For Vim version 9.1.  Last change: 2024 Apr 04
 
 		     VIM USER MANUAL - by Bram Moolenaar
 
@@ -649,13 +649,9 @@
 make the default file skip the settings.
 							*:CompilerSet*
 The second mechanism is to use ":set" for ":compiler!" and ":setlocal" for
-":compiler".  Vim defines the ":CompilerSet" user command for this.  However,
-older Vim versions don't, thus your plugin should define it then.  This is an
-example: >
+":compiler".  Vim defines the ":CompilerSet" user command for this.  This is
+an example: >
 
-  if exists(":CompilerSet") != 2
-    command -nargs=* CompilerSet setlocal <args>
-  endif
   CompilerSet errorformat&		" use the default 'errorformat'
   CompilerSet makeprg=nmake