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/compiler/irix5_cpp.vim b/runtime/compiler/irix5_cpp.vim
index 0112dc1..85ab8c6 100644
--- a/runtime/compiler/irix5_cpp.vim
+++ b/runtime/compiler/irix5_cpp.vim
@@ -2,6 +2,7 @@
 " Compiler:	SGI IRIX 5.3 CC or NCC
 " Maintainer:	David Harrison <david_jr@users.sourceforge.net>
 " Last Change:	2012 Apr 30
+"		2024 Apr 03 by The Vim Project (removed :CompilerSet definition)
 
 if exists("current_compiler")
   finish
@@ -10,10 +11,6 @@
 let s:keepcpo= &cpo
 set cpo&vim
 
-if exists(":CompilerSet") != 2		" older Vim always used :setlocal
-  command -nargs=* CompilerSet setlocal <args>
-endif
-
 CompilerSet errorformat=%E\"%f\"\\,\ line\ %l:\ error(%n):\ ,
 		    \%E\"%f\"\\,\ line\ %l:\ error(%n):\ %m,
 		    \%W\"%f\"\\,\ line\ %l:\ warning(%n):\ %m,