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/data/config.el

@@ -31,8 +31,6 @@
 (def-package! json-mode
   :mode "\\.js\\(?:on\\|[hl]int\\(rc\\)?\\)\\'"
   :config
-  (when (featurep! :feature syntax-checker)
-    (add-hook 'json-mode-hook #'flycheck-mode))
   (set-electric! 'json-mode :chars '(?\n ?: ?{ ?})))
 
 (def-package! vimrc-mode