Remove usage of no-op macro DEFINE_WEB_STATE_USER_DATA_KEY.

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

This CL was uploaded by git cl split.

R=kkhorimoto@chromium.org

Bug: 589840
Change-Id: I23e975d07b48897c6c10949a9fde8f9053356f87
Reviewed-on: https://chromium-review.googlesource.com/c/1351077Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611169}

ios/chrome/browser/voice/voice_search_navigations_tab_helper.mm

@@ -16,8 +16,6 @@
 #error "This file requires ARC support."
 #endif
 
-DEFINE_WEB_STATE_USER_DATA_KEY(VoiceSearchNavigationTabHelper);
-
 namespace {
 // The key under which the VoiceSearchNavigationMarkers are stored.
 const void* const kNavigationMarkerKey = &kNavigationMarkerKey;