Remove usage of no-op macro DEFINE_WEB_STATE_USER_DATA_KEY.

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

This CL was uploaded by git cl split.

R=rohitrao@chromium.org

Bug: 589840
Change-Id: I6b33f5491423d55455e912ee01adec2c73686122
Reviewed-on: https://chromium-review.googlesource.com/c/1351079Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612311}

ios/chrome/browser/tabs/legacy_tab_helper.mm

@@ -10,8 +10,6 @@
 #error "This file requires ARC support."
 #endif
 
-DEFINE_WEB_STATE_USER_DATA_KEY(LegacyTabHelper);
-
 // static
 void LegacyTabHelper::CreateForWebState(web::WebState* web_state) {
   CreateForWebStateInternal(web_state, nil);