Bluetooth: Remove unnecessary h5_build_pkt function

The implementation of h5_build_packet can be moved into
h5_prepare_pkt since all h5_prepare_pkt does is determine whether the
packet is reliable and then call h5_build_packet.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>