do not return real modifiable value for the cleanup (IDEA-170602)

plugins/gradle/src/org/jetbrains/plugins/gradle/settings/GradleExtensionsSettings.java

@@ -69,7 +69,7 @@ public class GradleExtensionsSettings implements PersistentStateComponent<Gradle
   public Settings getState() {
     // cleanup GradleExtensions entries created by previous version from workspace file
     // TODO remove PersistentStateComponent implementation in future version
-    return myState;
+    return new Settings();
   }
 
   @Override