Bluetooth: Audio: Remove enc_state from scan_delegator_set_bis_sync_state

The field is added/modified by bt_bap_scan_delegator_add_src
and bt_bap_scan_delegator_mod_src. This makes it easier
to modify the BIG sync state, without worrying about
the encryption state.

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