Move custom mouse event handler into MouseStateService

Don't need to keep it in CoreBrowserTerminal

src/browser/TestUtils.test.ts

@@ -393,10 +393,6 @@ export class MockMouseService implements IMouseService {
     throw new Error('Not implemented');
   }
 
-  public setCustomWheelEventHandler(): void {
-    throw new Error('Not implemented');
-  }
-
   public bindMouse(): void {
     throw new Error('Not implemented');
   }