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/cs.vim b/runtime/compiler/cs.vim
index 4f6dd3c..123dff8 100644
--- a/runtime/compiler/cs.vim
+++ b/runtime/compiler/cs.vim
@@ -3,6 +3,7 @@
 " Maintainer:             Yichao Zhou (broken.zhou@gmail.com)
 " Previous Maintainer:    Joseph H. Yao (hyao@sina.com)
 " Last Change:            Jul 22, 2019
+"                         2024 Apr 03 by The Vim Project (removed :CompilerSet definition)
 
 if exists("current_compiler")
   finish
@@ -11,10 +12,6 @@
 let s:keepcpo= &cpo
 set cpo&vim
 
-if exists(":CompilerSet") != 2		" older Vim always used :setlocal
-  command -nargs=* CompilerSet setlocal <args>
-endif
-
 CompilerSet errorformat&
 CompilerSet errorformat+=%f(%l\\,%v):\ %t%*[^:]:\ %m,
             \%trror%*[^:]:\ %m,