JS: Drop native map get/set intrinsics altogether.

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

js/js.libraries/src/core/core.kt

@@ -7,7 +7,6 @@ public external fun eval(expr: String): dynamic = noImpl
 
 public external val undefined: Nothing? = noImpl
 
-inline operator fun <K, V> MutableMap<K, V>.set(key: K, value: V): V? = asDynamic().set(key, value)
 
 @library
 public fun println() {}