health.vim: remove :CheckHealth command

For back-compat, :CheckHealth runs :checkhealth. But don't define
:CheckHealth explicitly, it adds noise to wildmenu completion.

Completion of healthchecks doesn't yet work with :checkhealth, this is
a regression but it needs to be implemented for :checkhealth rather than
keeping :CheckHealth around.

runtime/autoload/provider/node.vim

@@ -114,7 +114,7 @@ let s:err = ''
 let s:prog = provider#node#Detect()
 
 if empty(s:prog)
-  let s:err = 'Cannot find the "neovim" node package. Try :CheckHealth'
+  let s:err = 'Cannot find the "neovim" node package. Try :checkhealth'
 endif
 
 call remote#host#RegisterPlugin('node-provider', 'node', [])

runtime/plugin/health.vim

@@ -1,8 +1 @@
-function! s:complete(lead, _line, _pos) abort
-  return sort(filter(map(globpath(&runtimepath, 'autoload/health/*', 1, 1),
-        \ 'fnamemodify(v:val, ":t:r")'),
-        \ 'empty(a:lead) || v:val[:strlen(a:lead)-1] ==# a:lead'))
-endfunction
-
-command! -nargs=* -complete=customlist,s:complete CheckHealth
-      \ call health#check([<f-args>])
+autocmd CmdUndefined CheckHealth checkhealth