[Platform API] Clean-up some usages of Platform

Mostly unused imports. Also, in some places,
TargetPlatform/MultiTargetPlatform were just passed around without
actually using (e.g. in deserialization)

js/js.frontend/src/org/jetbrains/kotlin/js/resolve/JsResolverForModuleFactory.kt

@@ -84,7 +84,4 @@ object JsResolverForModuleFactory : ResolverForModuleFactory() {
 
         return ResolverForModule(packageFragmentProvider, container)
     }
-
-    override val targetPlatform: TargetPlatform
-        get() = JsPlatform
 }