JS: cache PropertyMetadata objects in property delegates

Also, remove unused PropertyMetadata instances

See KT-20737, KT-20738

js/js.translator/testData/box/delegateProperty/unusedPropertyMetadata.kt

@@ -0,0 +1,12 @@
+// EXPECTED_REACHABLE_NODES: 1131
+// PROPERTY_NOT_USED: PropertyMetadata
+import kotlin.reflect.KProperty
+
+class MyDelegate(val value: String) {
+    inline operator fun getValue(receiver: Any?, property: KProperty<*>): String = value
+}
+
+fun box(): String {
+    val x by MyDelegate("OK")
+    return x
+}