unpack_dl_tti_ssb_pdu_rel15_value(): match pack function

unpack_dl_tti_ssb_pdu_rel15_value() reads two additional bytes,
seemingly for RX power information. However, table 3-79 does not
contain such info, so I remove to match the corresponding pack function
pack_dl_tti_ssb_pdu_rel15_value().

TODO: tx power info relevant?