Label abbreviation feature is dead; kill it

Note: because this feature wasn't used at all, there is no need for
new schema version with automagic gerrit.config clean up.

Bug: issue 2618
Change-Id: Ic129ca43c21c1955b842d61b84c515d622711ced

gerrit-server/src/main/java/com/google/gerrit/server/query/change/EqualsLabelPredicate.java

@@ -103,12 +103,6 @@ class EqualsLabelPredicate extends IndexPredicate<ChangeData> {
         return lt;
       }
     }
-
-    for (LabelType lt : types.getLabelTypes()) {
-      if (toFind.equalsIgnoreCase(lt.getAbbreviation())) {
-        return lt;
-      }
-    }
     return null;
   }