Remove usage of no-op macro DEFINE_WEB_STATE_USER_DATA_KEY.

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

This CL was uploaded by git cl split.

R=olivierrobin@chromium.org

Bug: 589840
Change-Id: I47b0535e525d6ad8c442f0ac2953324d979c1cd2
Reviewed-on: https://chromium-review.googlesource.com/c/1351075Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611218}

ios/chrome/browser/metrics/ukm_url_recorder.mm

@@ -163,6 +163,3 @@ SourceId GetSourceIdForWebStateDocument(web::WebState* web_state) {
 }
 
 }  // namespace ukm
-
-DEFINE_WEB_STATE_USER_DATA_KEY(
-    ukm::internal::SourceUrlRecorderWebStateObserver);