feat: Always hide archive/unarchive action from toolbar

That streamlines the usability because the icons do not jump around when
there is more than one habit selected.

References: #465

uhabits-android/src/main/res/menu/list_habits_selection.xml

@@ -34,12 +34,12 @@
     <item
         android:id="@+id/action_archive_habit"
         android:title="@string/archive"
-        android:icon="?iconArchive" />
+        app:showAsAction="never" />
 
     <item
         android:id="@+id/action_unarchive_habit"
         android:title="@string/unarchive"
-        android:icon="?iconUnarchive"/>
+        app:showAsAction="never"/>
 
     <item
         android:id="@+id/action_delete"