Remove usage of no-op macro DEFINE_WEB_STATE_USER_DATA_KEY.

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

This CL was uploaded by git cl split.

R=mrefaat@chromium.org

Bug: 589840
Change-Id: I38d1ef88b752deda3e77f02fdb7208ddc13cd214
Reviewed-on: https://chromium-review.googlesource.com/c/1351964Reviewed-by: Mohammad Refaat <mrefaat@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611314}

ios/chrome/browser/itunes_urls/itunes_urls_handler_tab_helper.mm

@@ -28,8 +28,6 @@
 #error "This file requires ARC support."
 #endif
 
-DEFINE_WEB_STATE_USER_DATA_KEY(ITunesUrlsHandlerTabHelper);
-
 namespace {
 
 // The domain for iTunes appstore URLs.