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: vim/vim#14399408281e16a
Co-authored-by: Doug Kearns <dougkearns@gmail.com>