Remove search::GetSearchTerms since it always returns empty string

components/metrics/proto/omnibox_event.proto

Index: components/metrics/proto/omnibox_event.proto
diff --git a/components/metrics/proto/omnibox_event.proto b/components/metrics/proto/omnibox_event.proto
index 5d4f71996572cb7ab5846302be1f534db214e3b9..5fa208ee3924a30bb317acdd6c2b29abae2b2e08 100644
--- a/components/metrics/proto/omnibox_event.proto
+++ b/components/metrics/proto/omnibox_event.proto
@@ -111,10 +111,8 @@ message OmniboxEventProto {
     // 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.