Remove unimplemented functions

The TypeScript checker complain about it.
It was a workaround for closure-compiler, it's not needed anymore.

src/ol/format/MVT.js

@@ -287,35 +287,6 @@ class MVT extends FeatureFormat {
     this.layers_ = layers;
   }
 
-  /**
-   * Not implemented.
-   * @override
-   */
-  readFeature() {}
-
-  /**
-   * Not implemented.
-   * @override
-   */
-  readGeometry() {}
-
-  /**
-   * Not implemented.
-   * @override
-   */
-  writeFeature() {}
-
-  /**
-   * Not implemented.
-   * @override
-   */
-  writeGeometry() {}
-
-  /**
-   * Not implemented.
-   * @override
-   */
-  writeFeatures() {}
 }