fix(firebase-messaging): adjust deleteToken/removeToken signature

fix token remove function name according to https://github.com/chemerisuk/cordova-plugin-firebase-messaging#deletetoken

src/@ionic-native/plugins/firebase-messaging/index.ts

@@ -183,7 +183,7 @@ export class FirebaseMessaging extends IonicNativePlugin {
    * @returns {Promise<null>}
    */
   @Cordova({ sync: true })
-  revokeToken(): Promise<null> {
+  deleteToken(): Promise<null> {
     return;
   }
 }