Bluetooth: Audio: Rename bt_micp_aics functions

The AICS functionality is also possible via the bt_aics
API, and this was just a way to ensure that the AICS
instance was part of a MICS.

Removing makes MICP much simpler, and we can expect user
to know the context of a AICS.

This also fixes a few bad/missing guards.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>