Remove unused def

[sogaiu]

This PR is a suggestion to remove an apparently unused local def in boot.janet:

(defn flycheck
  # docstring elided
  [path &keys kwargs]
  (def mc @{})        # <-- this def
  (def new-env (make-env (get kwargs :env)))

It looks like it might have shown up in the recent refactoring of flycheck machinery in this commit.

[pepe]

LGTM