refactor: remove camera duplicate code

Author: GuoLei1990

packages/core/src/Camera.ts

@@ -621,10 +621,6 @@ export class Camera extends Component {
     this._viewport._onValueChanged = null;
     this.engine.canvas._sizeUpdateFlagManager.removeListener(this._onPixelViewportChanged);
 
-    //@ts-ignore
-    this._viewport._onValueChanged = null;
-    this.engine.canvas._sizeUpdateFlagManager.removeListener(this._onPixelViewportChanged);
-
     this._entity = null;
     this._globalShaderMacro = null;
     this._frustum = null;