Remove the callable? predicate.

Many times are callable now in some circumstances, so
the predicate is not that useful.

src/core/core.janet

@@ -104,9 +104,6 @@
 (defn indexed? "Check if x is an array or tuple." [x]
   (def t (type x))
   (if (= t :array) true (= t :tuple)))
-(defn callable? "Check if x is a function or cfunction." [x]
-  (def t (type x))
-  (if (= t :function) true (= t :cfunction)))
 (defn true? "Check if x is true." [x] (= x true))
 (defn false? "Check if x is false." [x] (= x false))
 (defn nil? "Check if x is nil." [x] (= x nil))