Remove unneeded config-checks (#586)

Configlet now handles these itself.

This leaves CONFIG-CHECKER with no checks; but I currently think it is
good to keep it in place even being a no-op at the moment. There is
still the possibility of track specific checks (for config or
otherwise) that could be done in this framework.

Author: verdammelt

src/config-checker.asd

@@ -13,8 +13,7 @@
                (:file "utils")
                (:file "main")
                (:module "checkers"
-                :components ((:file "each-concept-has-a-directory")
-                             (:file "exercise-concepts-are-in-concept-list"))))
+                :components ()))
 
   :perform (test-op (o c)
                     (declare (ignore o c))