feat!: remove vimballs (#22402)

Vimball is an outdated feature that is rarely used these days. It is not
a maintenance burden on its own, but it is nonetheless dead weight and
something we'd need to tell users to ignore when they inevitably ask
what it is.

See: https://github.com/neovim/neovim/pull/21369#issuecomment-1347615173

runtime/doc/vim_diff.txt

@@ -573,6 +573,7 @@ Commands:
   :cscope
   :lcscope
   :scscope
+  :Vimuntar
 
 Compile-time features:
   Emacs tags support
@@ -764,5 +765,8 @@ Hardcopy:
   `:hardcopy` was removed. Instead, use `:TOhtml` and print the resulting HTML
   using a web browser or some other HTML viewer.
 
+Bundled plugins:
+  vimball                                                             *vimball*
+
 ==============================================================================
  vim:tw=78:ts=8:sw=2:et:ft=help:norl: