Widgets: Remove option to create StackWidgets

StackWidgets have been unfortunately been very unreliable on multiple phones,
and fixing it does not appear to be simple. This commit removes the ability
to create new StackWidgets, but existing ones should remain functional.

uhabits-android/src/main/res/layout/widget_configure_activity.xml

@@ -30,10 +30,4 @@
         android:layout_weight="1">
     </ListView>
 
-    <Button
-        android:id="@+id/buttonSave"
-        android:layout_height="wrap_content"
-        android:layout_width="match_parent"
-        android:text="@string/save"/>
-
 </LinearLayout>