Remove usage of no-op macro DEFINE_WEB_STATE_USER_DATA_KEY.

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

This CL was uploaded by git cl split.

R=rohitrao@chromium.org

Bug: 589840
Change-Id: Ia6e2ae8b3dc9334e9cec18328d7dda086f7ab57a
Reviewed-on: https://chromium-review.googlesource.com/c/1351961Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612313}

ios/chrome/browser/sessions/ios_chrome_session_tab_helper.mm

@@ -8,8 +8,6 @@
 #error "This file requires ARC support."
 #endif
 
-DEFINE_WEB_STATE_USER_DATA_KEY(IOSChromeSessionTabHelper);
-
 IOSChromeSessionTabHelper::IOSChromeSessionTabHelper(web::WebState* web_state)
     : session_id_(SessionID::NewUnique()),
       window_id_(SessionID::InvalidValue()) {}