Remove unimplemented getClientRects method from ReadOnlyElement (facebook#39348)

Summary:
Pull Request resolved: facebook#39348

We removed this method from the proposal, so we don't need to keep the method around unimplemented.

Changelog: [internal]

Reviewed By: NickGerleman

Differential Revision: D49069518

fbshipit-source-id: c6b30e2ccbaaadfd5558bb5535e2530affa72273

Author: rubennorte

packages/react-native/Libraries/DOM/Nodes/ReadOnlyElement.js

@@ -227,10 +227,6 @@ export default class ReadOnlyElement extends ReadOnlyNode {
     return new DOMRect(0, 0, 0, 0);
   }
 
-  getClientRects(): DOMRectList {
-    throw new TypeError('Unimplemented');
-  }
-
   /**
    * Pointer Capture APIs
    */