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/cargo.vim b/runtime/compiler/cargo.vim
index aa9b01e..cae2e51 100644
--- a/runtime/compiler/cargo.vim
+++ b/runtime/compiler/cargo.vim
@@ -2,6 +2,7 @@
 " Compiler:         Cargo Compiler
 " Maintainer:       Damien Radtke <damienradtke@gmail.com>
 " Latest Revision:  2023-09-11
+"                   2024 Apr 03 by The Vim Project (removed :CompilerSet definition)
 " For bugs, patches and license go to https://github.com/rust-lang/rust.vim
 
 if exists('current_compiler')