Bluetooth: BAP: Use def_bool instead of select for BT_BAP_UNICAST

Remove the selects from BT_BAP_UNICAST_SERVER and BT_BAP_UNICAST_CLIENT
and use a def_bool for BT_BAP_UNICAST.

This is part of an effort to reduce select in Kconfig.

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