Remove usage of no-op macro DEFINE_WEB_STATE_USER_DATA_KEY.

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

This CL was uploaded by git cl split.

R=pkl@chromium.org

Bug: 589840
Change-Id: I3eb75e5a3316c7da05abab6e99b3a727c4f92ce4
Reviewed-on: https://chromium-review.googlesource.com/c/1351958Reviewed-by: Peter Lee <pkl@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611168}

ios/chrome/browser/store_kit/store_kit_tab_helper.mm

@@ -10,8 +10,6 @@
 #error "This file requires ARC support."
 #endif
 
-DEFINE_WEB_STATE_USER_DATA_KEY(StoreKitTabHelper);
-
 StoreKitTabHelper::StoreKitTabHelper(web::WebState* web_state) {}
 
 StoreKitTabHelper::~StoreKitTabHelper() {}