RN: Delete EventEmitter#removeSubscription

Summary:
Deletes `EventEmitter#removeSubscription`, which has been deprecated since D27704279 (cb6cbd1).

Relatedly, the `removeListener` family of methods (which were deprecated ad the same time) were recently removed by D35549719 (2596b2f).

Changelog:
[General][Removed] - Removed `EventEmitter.prototype.removeSubscription` method.

Reviewed By: christophpurrer

Differential Revision: D36171048

fbshipit-source-id: 2409d235d43049cddfe0a54bcc60e1f47d4185c5

Author: yungsters

Libraries/vendor/emitter/_EventEmitter.js

@@ -78,19 +78,6 @@ class EventEmitter<EventDefinitions: {...}> {
     this._subscriber.removeAllSubscriptions(eventType);
   }
 
-  /**
-   * @deprecated Use `remove` on the EventSubscription from `addListener`.
-   */
-  removeSubscription<K: $Keys<EventDefinitions>>(
-    subscription: EmitterSubscription<EventDefinitions, K>,
-  ): void {
-    console.warn(
-      'EventEmitter.removeSubscription(...): Method has been deprecated. ' +
-        'Please instead use `remove()` on the subscription itself.',
-    );
-    this.__removeSubscription(subscription);
-  }
-
   /**
    * Called by `EmitterSubscription` to bypass the above deprecation warning.
    */