CLJS-1269: realized? docstring refers to promise and future

Since `promise` and `future` are not implemented in
ClojureScript, eliminate references to them in the
docstring for `realized?`

Author: mfikes

src/main/cljs/cljs/core.cljs

@@ -8883,7 +8883,7 @@ reduces them without incurring seq initialization"
     x))
 
 (defn ^boolean realized?
-  "Returns true if a value has been produced for a promise, delay, future or lazy sequence."
+  "Returns true if a value has been produced for a delay or lazy sequence."
   [d]
   (-realized? d))