Integrate nr_fill_nfapi_dl_pdu() back into nr_schedule_ue_spec()

Initially, filling the PDCCH and PDSCH nFAPI messages was split into a
separate function (in an attempt to keep the code structure similar to
LTE). However, this proved as not helpful: the nr_fill_nfapi_dl_pdu()
just filled the messages, with a parameter list almost size as long as
the actual messages (because most parameters are kind of independent).
This made no sense, so we put it back.

Also, from an understanding POV, they just fill a message as specified
in SCF222, so it should not be a problem.