Add missing delete to requestIdleCallback

Summary:
This line accidentally became a no-op when the Map was converted to an Object.
Closes https://github.com/facebook/react-native/pull/15891

Differential Revision: D5811069

Pulled By: TheSavior

fbshipit-source-id: 43ac1835d15e2bee67ee45646bc238f917013836

Libraries/Core/Timers/JSTimers.js

@@ -331,7 +331,7 @@ const JSTimers = {
             const timeoutId = requestIdleCallbackTimeouts[id];
             if (timeoutId) {
               JSTimers.clearTimeout(timeoutId);
-              requestIdleCallbackTimeouts[id];
+              delete requestIdleCallbackTimeouts[id];
             }
             return func(deadline);
           }