refactor: get rid of diagnostic message

This is in response to this feedback:

    #393 (comment)

The user in question is loading Command-T like this, using packer:

    use {
      'wincent/command-t',
      run = 'cd lua/wincent/commandt/lib && make',
      config = function()
        vim.g.CommandTPreferredImplementation = 'lua'
        require('wincent.commandt').setup({})
      end
    }

and they see this whenever they run `:PackerCompile`:

    commandt.setup():
      `commandt.setup()` was called after Ruby plugin setup has already run

So, I'm removing this check as it doesn't add a lot of value. My advice,
then, is to move this statement:

    vim.g.CommandTPreferredImplementation = 'lua'

up into the top-level of the `~/.config/nvim/init.lua`. That way,
whenever Command-T's `plugin/command-t.vim` is sourced, it will do the
right thing, which means:

1. Skip doing anything if `command_t_loaded` is already set.
2. Do Lua-centric set-up instead of Ruby-centric set-up.

And then when `plugin/command-t.lua` is sourced (which will always
happen second, because Neovim loads Lua _after_ Vimscript), it will
again do the right thing (ie. do the Lua-centric instead of Ruby-centric
set-up).

lua/wincent/commandt/init.lua

@@ -178,13 +178,7 @@ end
 commandt.setup = function(options)
   local errors = {}
 
-  if vim.g.command_t_loaded == 1 then
-    -- May not be an error if you (for whatever reason) are calling `setup()`
-    -- twice (ie. later on during a session), but it's presumed that if you're
-    -- doing that, you know enough about what you're doing not to understand
-    -- that this error message is nothing to worry about.
-    table.insert(errors, '`commandt.setup()` was called after Ruby plugin setup has already run')
-  elseif vim.g.CommandTPreferredImplementation == 'ruby' then
+  if vim.g.CommandTPreferredImplementation == 'ruby' then
     table.insert(errors, '`commandt.setup()` was called, but `g:CommandTPreferredImplementation` is set to "ruby"')
   else
     vim.g.CommandTPreferredImplementation = 'lua'