Remove is_active property from SimpleInterfaceMonitor

Remove is_active property from SimpleInterfaceMonitor

is_active property from SimpleInterfaceMonitor shadows
the method is_active inherited from AsyncProcess.
The property checks that ovsdb monitor is running and
that it received some data. When ovsdb monitor starts
it always receives data, since it processes the interfaces
present on the machine, so the flag data_received will
always be set to true right after SimpleInterfaceMonitor
starts. Considering that, is_active can be removed and
the method is_active inherited from AsyncProcess can be
used instead.

Change-Id: I05faeddd061ab45af51c044a10462c3a57593d4d