RN: Delete nullthrows Import in MutationObserver (#51672)

Summary:
Pull Request resolved: #51672

Deletes an unused import.

Changelog:
[Internal]

Reviewed By: javache

Differential Revision: D75583225

fbshipit-source-id: be57005f5647234acf450c13345987da24e92696

Author: yungsters

packages/react-native/src/private/webapis/mutationobserver/MutationObserver.js

@@ -16,7 +16,6 @@ import type MutationRecord from './MutationRecord';
 import ReactNativeElement from '../dom/nodes/ReactNativeElement';
 import {setPlatformObject} from '../webidl/PlatformObjects';
 import * as MutationObserverManager from './internals/MutationObserverManager';
-import nullthrows from 'nullthrows';
 
 export type MutationObserverCallback = (
   mutationRecords: $ReadOnlyArray<MutationRecord>,