Drop legacy device types.

powervr_device is obsoleted by the more general gpu_device.
akm_device and accelerometer_device are obsoleted by the more
general sensors_device.

We could also drop the file_contexts entries altogether and
take them to device-specific policy (in this case, they all
came from crespo, so that is obsolete for master).

Change-Id: I63cef43b0d66bc99b80b64655416cc050f443e7d
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>