Remove usage of no-op macro DEFINE_WEB_STATE_USER_DATA_KEY.

This CL removes macro usage from /ios/chrome/browser/reading_list.

This CL was uploaded by git cl split.

R=olivierrobin@chromium.org

Bug: 589840
Change-Id: Ia0b05a38e3daa883a1ebb3d2255df84d9f30eb45
Reviewed-on: https://chromium-review.googlesource.com/c/1351953Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611229}

ios/chrome/browser/reading_list/reading_list_web_state_observer.mm

@@ -22,8 +22,6 @@
 #error "This file requires ARC support."
 #endif
 
-DEFINE_WEB_STATE_USER_DATA_KEY(ReadingListWebStateObserver);
-
 // static
 void ReadingListWebStateObserver::CreateForWebState(
     web::WebState* web_state,