presence: Eliminate unused mobile-related code.

We had a plan at some point to use this to display a phone icon or
something for users who would receive push notifications if you
messaged them.  IT's not clear that feature was a good idea in any
case, but it certainly shouldn't be synced as presence data; it would
change >100x less often than the rest of presence and so should likely
be synced differently, maybe as a property on user. So it's best to
delete this prototype.

frontend_tests/node_tests/activity.js

@@ -806,7 +806,7 @@ run_test('update_presence_info', () => {
     activity.update_presence_info(alice.user_id, info, server_time);
     assert(inserted);
 
-    const expected = { status: 'active', mobile: false, last_active: 500 };
+    const expected = { status: 'active', last_active: 500 };
     assert.deepEqual(presence.presence_info[alice.user_id], expected);
 });