Change flycheck initialization strategy

Initialize it globally and turn it off where needed, instead of enabling
it on demand. Also fixes void-function: flycheck-mode errors when
:feature syntax-checker is disabled. This is experimental.

Indirectly fixes #710

modules/lang/python/config.el

@@ -22,7 +22,7 @@ is loaded.")
         python-indent-guess-indent-offset-verbose nil
         python-shell-interpreter "python")
   :config
-  (add-hook! 'python-mode-hook #'(flycheck-mode highlight-numbers-mode))
+  (add-hook 'python-mode-hook #'highlight-numbers-mode)
 
   (set-env! "PYTHONPATH" "PYENV_ROOT")
   (set-electric! 'python-mode :chars '(?:))