Remove usage of no-op macro DEFINE_WEB_STATE_USER_DATA_KEY.

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

This CL was uploaded by git cl split.

R=rohitrao@chromium.org

Bug: 589840
Change-Id: I9741755b7ed101a5baf4e6bacc38abd7ad0a4029
Reviewed-on: https://chromium-review.googlesource.com/c/1351955Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612315}

ios/chrome/browser/find_in_page/find_tab_helper.mm

@@ -18,8 +18,6 @@ const char kFindActionName[] = "Find";
 const char kFindNextActionName[] = "FindNext";
 const char kFindPreviousActionName[] = "FindPrevious";
 
-DEFINE_WEB_STATE_USER_DATA_KEY(FindTabHelper);
-
 FindTabHelper::FindTabHelper(web::WebState* web_state) {
   web_state->AddObserver(this);
   controller_ = [[FindInPageController alloc] initWithWebState:web_state];