fix(runtime/genvimvim): omit s[ubstitute] from vimCommand #18480

It's special cased by the vimSubst syntax group, and isn't present in Vim's
vimCommand group.

For example, this fixes `call s:Foo()` highlighting `:` as Error in Nvim, as the
`s` is parsed as vimCommand rather than as vimUserFunc since
`contains=vimCommand` was added to vimUserFunc (and vimFunc) in a rt update.

Interestingly, `g:`, `l:`, etc. have the same issues due to :global, :list, etc.
Vim also has that problem, so it should ideally be fixed upstream.

We could also omit g[lobal] from vimCommand and rely on vimGlobal instead, but
it doesn't work in some cases, like when there's a `:` before the command. Also,
Vim matches only `g` in vimCommand for some reason, which doesn't produce any
highlight for `:global/foo/bar` (with Nvim you at least get some highlights on
the `global` bit despite the leading `:`).

Also, remove special handling of :py3 in syntax/vim.vim, as the generator seems
to have no problems finding it.

runtime/syntax/vim.vim

@@ -120,9 +120,6 @@ else
  com! -nargs=*	VimFoldt	<args>
 endif
 
-" commands not picked up by the generator (due to non-standard format) {{{2
-syn keyword vimCommand contained	py3
-
 " Deprecated variable options {{{2
 if exists("g:vim_minlines")
  let g:vimsyn_minlines= g:vim_minlines