Remove unused string reference.

Change-Id: I411a924c22c84d1a62e0e42df7429e4d869fe3ed
diff --git a/res/xml/sound_settings.xml b/res/xml/sound_settings.xml
index c0bca2e..d5f0e6b 100644
--- a/res/xml/sound_settings.xml
+++ b/res/xml/sound_settings.xml
@@ -30,7 +30,6 @@
     <ListPreference
             android:key="vibrate"
             android:title="@string/vibrate_title"
-            android:summary="@string/vibrate_summary"
             android:entries="@array/vibrate_entries"
             android:entryValues="@array/vibrate_values" />