Remove vestigial "Choose date format" option.

Almost all of our date formatting goes via icu these days, so this is
a no-op almost everywhere. (And should be a no-op everywhere soon.)
There's no obvious way to fix it, so remove it. UI guidelines say we
should use spelled out month names anyway.

Bug: 18322220
Change-Id: Ic9e903eceab3c096426f313bf4ef3a38644774e1

res/xml/date_time_prefs.xml

@@ -52,9 +52,4 @@
         android:title="@string/date_time_24hour"
         />
 
-    <ListPreference
-        android:key="date_format"
-        android:title="@string/date_time_date_format"
-        android:summary="mm/dd/yyyy"/>
-
 </PreferenceScreen>