Remove device_activity

[ianb]

The device_activity table is growing fast and we're not making use of the data in it.

We should remove the table, and remove all references to addDeviceActivity

[ianb]

For posterity, this is what we have in the table now:

pageshot=> select count(deviceid), event_type from device_activity group by event_type;
   count   |    event_type
-----------+------------------
 327471492 | login
        21 | invalid-register
      1737 | unload