Remove search::GetSearchTerms since it always returns empty string

components/metrics/proto/omnibox_event.proto

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Stores information about an omnibox interaction.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 option java_outer_classname = "OmniboxEventProtos";
@@ skipping 93 matching lines @@
     // classify the page into those categories, not HOME_PAGE.
     HOME_PAGE = 3;
 
     // The catch-all entry of everything not included somewhere else
     // on this list.
     OTHER = 4;
 
     // The instant new tab page enum value was deprecated on August 2, 2013.
     OBSOLETE_INSTANT_NTP = 5;
 
-    // The user is on a search result page that's doing search term
-    // replacement, meaning the search terms should've appeared in the omnibox
-    // before the user started editing it, not the URL of the page.
-    SEARCH_RESULT_PAGE_DOING_SEARCH_TERM_REPLACEMENT = 6;
+    // The search term replacement enum value was deprecated in August 2016.
+    OBSOLETE_SEARCH_RESULT_PAGE_DOING_SEARCH_TERM_REPLACEMENT = 6;
 
     // The new tab page in which this omnibox interaction first started
     // with the user having focus in the omnibox.
     INSTANT_NTP_WITH_OMNIBOX_AS_STARTING_FOCUS = 7;
 
     // The new tab page in which this omnibox interaction first started
     // with the user having focus in the fakebox.
     INSTANT_NTP_WITH_FAKEBOX_AS_STARTING_FOCUS = 8;
 
     // The user is on a search result page that's not doing search term
@@ skipping 149 matching lines @@
     // The number of times this provider returned a non-zero number of
     // suggestions during this omnibox session.
     // Note that each provider may define a session differently for its
     // purposes.
     optional int32 times_returned_results_in_session = 5;
   }
   // A list of diagnostic information about each provider.  Providers
   // will appear at most once in this list.
   repeated ProviderInfo provider_info = 12;
 }