Remove Activity Recognition from nanohub sensorhal

The connection to the AR HAL was severed in Android Oreo.
This change simply removes all unused references.

Test: Build compiles. Ensure that clients that provide AR through other
channels continue to work.
Signed-off-by: Ben Fennema <fennema@google.com>

Change-Id: Id0b1eaf3e67aa3ec81d8fb81cdddd2109250a990