Remove usage of no-op macro DEFINE_WEB_STATE_USER_DATA_KEY.

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

This CL was uploaded by git cl split.

R=rohitrao@chromium.org

Bug: 589840
Change-Id: I4b9a1e4e554819b741e0901208665dc7142db08e
Reviewed-on: https://chromium-review.googlesource.com/c/1351074Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612308}

ios/chrome/browser/infobars/infobar_manager_impl.mm

@@ -21,8 +21,6 @@
 #error "This file requires ARC support."
 #endif
 
-DEFINE_WEB_STATE_USER_DATA_KEY(InfoBarManagerImpl);
-
 namespace {
 
 infobars::InfoBarDelegate::NavigationDetails