Back out "[react-native][PR] Allow Animation EndResult callback to return Promise"

Summary:
Original commit changeset: 420d29d262b6

Reverts https://github.com/facebook/react-native/pull/25793 / D16515465

Union type property is not supported by codegen. We don't want to support unions yet and because the improvement is not that big and not yet published as stable for OSS (neither used anywhere internally) we can safely revert it.

Reviewed By: RSNara

Differential Revision: D16621228

fbshipit-source-id: 2fa416eef1ae353990860026ca97d2b0b429a852

Libraries/Animated/src/NativeAnimatedModule.js

@@ -14,7 +14,7 @@ import type {TurboModule} from '../../TurboModule/RCTExport';
 import * as TurboModuleRegistry from '../../TurboModule/TurboModuleRegistry';
 
 type EndResult = {finished: boolean};
-type EndCallback = (result: EndResult) => void | Promise<void>;
+type EndCallback = (result: EndResult) => void;
 
 export type EventMapping = {|
   nativeEventPath: Array<string>,

Libraries/Animated/src/animations/Animation.js

@@ -14,7 +14,7 @@ const NativeAnimatedHelper = require('../NativeAnimatedHelper');
 import type AnimatedValue from '../nodes/AnimatedValue';
 
 export type EndResult = {finished: boolean};
-export type EndCallback = (result: EndResult) => void | Promise<void>;
+export type EndCallback = (result: EndResult) => void;
 
 export type AnimationConfig = {
   isInteraction?: boolean,