JS: Drop native map get/set intrinsics altogether.

MutableMap.set extension operator is now common among stdlib and stdlib-js.
#KT-2323

js/js.translator/testData/box/native/kt2323.js

@@ -1 +0,0 @@
-var classes = {"answer": 42}, classesMutable = {};

js/js.translator/testData/box/native/kt2323.kt

@@ -1,11 +0,0 @@
-package foo
-
-
-external val classes: Map<String, Any> = noImpl
-
-external val classesMutable: HashMap<String, String> = noImpl
-
-fun box(): String {
-    classesMutable.set("why", "?")
-    return if (classes.get("answer") == 42 && classesMutable.get("why") == "?") "OK" else "fail"
-}