fire: (FileReader) remove unused _subscriptions attribute (#33946)

Summary:
I was reading source code, and I noticed the _subscriptions attribute (and the _clearSubscriptions method) of the FileReader is not used.

## Changelog

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[Internal] [Changed] - Remove unused _subscriptions attribute in FileReader

Pull Request resolved: https://github.com/facebook/react-native/pull/33946

Test Plan: I checked that flow and jest are still green as I can't think about another way.

Reviewed By: lunaleaps

Differential Revision: D36807828

Pulled By: cortinico

fbshipit-source-id: 9bb599bbc7b79d2b4c010ba84cc8777e29b974ca

Libraries/Blob/FileReader.js

@@ -46,7 +46,6 @@ class FileReader extends (EventTarget(...READER_EVENTS): any) {
   _error: ?Error;
   _result: ?ReaderResult;
   _aborted: boolean = false;
-  _subscriptions: Array<any> = [];
 
   constructor() {
     super();
@@ -59,11 +58,6 @@ class FileReader extends (EventTarget(...READER_EVENTS): any) {
     this._result = null;
   }
 
-  _clearSubscriptions(): void {
-    this._subscriptions.forEach(sub => sub.remove());
-    this._subscriptions = [];
-  }
-
   _setReadyState(newState: ReadyState) {
     this._readyState = newState;
     this.dispatchEvent({type: 'readystatechange'});