Remove Vundle shim and deprecation warning

We've been using vim-plug over Vundle since 5724d12 (April 2015). Users
have had long enough to migrate their `vimrc.bundles.local` to use
vim-plug's `Plug` command.

vimrc.bundles

@@ -10,28 +10,6 @@ function! s:UnPlug(plug_name)
 endfunction
 command!  -nargs=1 UnPlug call s:UnPlug(<args>)
 
-" Shim command and function to allow migration from Vundle to vim-plug.
-function! VundleToPlug(vundle_command, arg, ...)
-  echom "You are using Vundle's `".a:vundle_command."` command to declare plugins. Dotfiles now uses vim-plug for plugin management. Please rename uses of `".a:vundle_command."` to `Plug`. Plugin was '".a:arg."'."
-  let vim_plug_options = {}
-
-  if a:0 > 0
-    if has_key(a:1, 'name')
-      let name = a:1.name
-      let vim_plug_options.dir = "$HOME/.vim/bundle/".a:1.name
-    endif
-
-    if has_key(a:1, 'rtp')
-      let vim_plug_options.rtp = a:1.rtp
-    endif
-  endif
-
-  Plug a:arg, vim_plug_options
-endfunction
-
-com! -nargs=+  -bar Plugin call VundleToPlug("Plugin", <args>)
-com! -nargs=+  -bar Bundle call VundleToPlug("Bundle", <args>)
-
 let g:has_async = v:version >= 800 || has('nvim')
 
 call plug#begin('~/.vim/bundle')