Remove usage of no-op macro DEFINE_WEB_STATE_USER_DATA_KEY.

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

This CL was uploaded by git cl split.

R=eugenebut@chromium.org

Bug: 589840
Change-Id: I047edb54ce7991b4aefd3af99a89affaa53f7a56
Reviewed-on: https://chromium-review.googlesource.com/c/1351956Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611248}

ios/chrome/browser/webui/net_export_tab_helper.mm

@@ -12,8 +12,6 @@
 #error "This file requires ARC support."
 #endif
 
-DEFINE_WEB_STATE_USER_DATA_KEY(NetExportTabHelper);
-
 // static
 void NetExportTabHelper::CreateForWebState(
     web::WebState* web_state,