Remove getCategoryKey() from DashboardFragment children.

Use the centralized registry to look up category key instead.

Bug: 32936784
Test: make RunSettingsRoboTests -j40
Change-Id: I0b8c72d70f93e4b5c58871ac90de41f69ad15653

src/com/android/settings/DateTimeSettings.java

@@ -55,11 +55,6 @@ public class DateTimeSettings extends DashboardFragment implements
         return MetricsEvent.DATE_TIME;
     }
 
-    @Override
-    protected String getCategoryKey() {
-        return null;
-    }
-
     @Override
     protected String getLogTag() {
         return TAG;