On the interface usage stats, filter out stats about server-owned interfaces

[davidebriani]

By design, Astarte currently accumulates stats about data sent by the device to the cloud.
However it does not compute stats about data sent from the cloud to the device.

This means that all server-owned interfaces will report 0 data transmitted, so considering those interfaces for displaying usage stats on the Dashboard only adds noise and it is not meaningful for the user.

We can improve the Device Stats section on the device page to:

• make it clear that those stats are only about data published by the device.
• remove superfluous elements that do not add value for the user.