Remove duplicate declaration

Summary:
Changelog: [internal]

This is already defined in Touch.h

Reviewed By: shergin

Differential Revision: D25242843

fbshipit-source-id: 23bac2a60f3d995e34d342c3a189760875f4bc77

ReactCommon/react/renderer/components/view/TouchEvent.h

@@ -16,8 +16,6 @@
 namespace facebook {
 namespace react {
 
-using Touches = std::unordered_set<Touch, Touch::Hasher, Touch::Comparator>;
-
 /*
  * Defines the `touchstart`, `touchend`, `touchmove`, and `touchcancel` event
  * types.