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/haskell/+dante.el

@@ -5,8 +5,7 @@
   :after haskell-mode
   :hook (haskell-mode . dante-mode)
   :config
-  (add-hook 'haskell-mode-hook #'interactive-haskell-mode)
-  (add-hook 'dante-mode-hook #'flycheck-mode))
+  (add-hook 'haskell-mode-hook #'interactive-haskell-mode))
 
 
 (def-package! company-ghc

modules/lang/haskell/+intero.el

@@ -13,7 +13,6 @@ This is necessary because `intero-mode' doesn't do its own error checks."
         (message "Couldn't find stack. Refusing to enable intero-mode."))))
   (add-hook 'haskell-mode-hook #'+haskell|init-intero)
   :config
-  (add-hook 'intero-mode-hook #'flycheck-mode)
   (set-lookup-handlers! 'intero-mode :definition #'intero-goto-definition))