fix SeekBarPreference support for summaries

Overriding getSummary breaks support for summaries even though it's
supported in the layout for SeekBarPreference. If there's no summary the
return value is already null in the inherited implementation.

Change-Id: I2e72ea3c4a624acbf200e9617173eb072572fa57

core/java/android/preference/SeekBarPreference.java

@@ -78,11 +78,6 @@ public class SeekBarPreference extends Preference
         seekBar.setEnabled(isEnabled());
     }
 
-    @Override
-    public CharSequence getSummary() {
-        return null;
-    }
-
     @Override
     protected void onSetInitialValue(boolean restoreValue, Object defaultValue) {
         setProgress(restoreValue ? getPersistedInt(mProgress)