Remove SubscribableMixin from SizeFlexibilityUpdateTest (#21580)

Summary:
Related to #21485

Remove SubscribableMixin from SizeFlexibilityUpdateTest

- All flow tests succeed.
- yarn run test-android-instrumentation && CI integration tests

[GENERAL] [ENHANCEMENT] [SizeFlexibilityUpdateTest.js] - rm SubscribableMixin
Pull Request resolved: https://github.com/facebook/react-native/pull/21580

Reviewed By: TheSavior

Differential Revision: D10276267

Pulled By: RSNara

fbshipit-source-id: fc89b43099c788cba560c9aaf0819cd5cfab38c3

IntegrationTests/SizeFlexibilityUpdateTest.js

@@ -5,6 +5,7 @@
  * LICENSE file in the root directory of this source tree.
  *
  * @format
+ * @flow
  */
 
 'use strict';
@@ -13,10 +14,10 @@ const React = require('react');
 const createReactClass = require('create-react-class');
 const ReactNative = require('react-native');
 const RCTNativeAppEventEmitter = require('RCTNativeAppEventEmitter');
-const Subscribable = require('Subscribable');
 const {View} = ReactNative;
 
 const {TestModule} = ReactNative.NativeModules;
+import type EmitterSubscription from 'EmitterSubscription';
 
 const reactViewWidth = 111;
 const reactViewHeight = 222;
@@ -25,16 +26,21 @@ let finalState = false;
 
 const SizeFlexibilityUpdateTest = createReactClass({
   displayName: 'SizeFlexibilityUpdateTest',
-  mixins: [Subscribable.Mixin],
+  _subscription: (null: ?EmitterSubscription),
 
   UNSAFE_componentWillMount: function() {
-    this.addListenerOn(
-      RCTNativeAppEventEmitter,
+    this._subscription = RCTNativeAppEventEmitter.addListener(
       'rootViewDidChangeIntrinsicSize',
       this.rootViewDidChangeIntrinsicSize,
     );
   },
 
+  componentWillUnmount: function() {
+    if (this._subscription != null) {
+      this._subscription.remove();
+    }
+  },
+
   markPassed: function() {
     TestModule.markTestPassed(true);
     finalState = true;