checkers/spell: replace flyspell with spell-fu

Spell-fu is significantly faster, but does produce more false
positives (more faces must be added to spell-fu-faces-exclude to reduce
these).

Unfortunately, there is no fancy "correction" interface for spell-fu
yet, so we'll have to resort to ispell-word (on z=) for now.

modules/lang/markdown/config.el

@@ -53,8 +53,6 @@ capture, the end position, and the output buffer.")
     (add-to-list 'org-src-lang-modes '("md" . markdown)))
 
   :config
-  (set-flyspell-predicate! '(markdown-mode gfm-mode)
-    #'+markdown-flyspell-word-p)
   (set-lookup-handlers! '(markdown-mode gfm-mode)
     ;; `markdown-follow-thing-at-point' may open an external program or a
     ;; buffer. No good way to tell, so pretend it's async.