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/DisplaySettings.java

@@ -67,11 +67,6 @@ public class DisplaySettings extends DashboardFragment {
         mProgressiveDisclosureMixin.setTileLimit(4);
     }
 
-    @Override
-    protected String getCategoryKey() {
-        return CategoryKey.CATEGORY_DISPLAY;
-    }
-
     @Override
     protected int getPreferenceScreenResId() {
         if (mDashboardFeatureProvider.isEnabled()) {