Note: after this commit, monolithic works but nFAPI operation is broken
and fixed in the next commit, to keep the changes small(er). All
simulators work.

This commit removes the use of msgDataTx's intermediate buffers for FAPI
messages and the call of the "Schedule response". The latter incurs an
additional copy of FAPI messages for the TX chain (for the RX chain, it
is and will remain there) which we can avoid.

Instead, it uses an NR_Sched_Rsp_t-typed variable sched_response to
store the results of the scheduler when called from tx_func(). Thus, the
scheduler remains unchanged (it gets a pointer to where to put FAPI
messages), but we don't call nr_schedule_response() to then copy FAPI
messages into the L1, but have them in a local variable sched_response.
sched_response on on the data segment, as it is big and would overflow
the stack; at the same time, this is ok because only one tx_func() runs
at a time.

Since we don't use allocate_sched_response() anymore, we don't need
deref_sched_response().

clear_slot_beamid() is moved to tx_func(), as it was called in
nr_schedule_response().

Since the "intermediate" NR_gNB_DLSCH_t structure groups both the
DL_tti_pdsch and TX_data.req structures, a pointer has been created to
point to pdsch_pdu. This affects a number of simulators, as they have to
put some messages on the stack:

- nr_dlschsim: put the corresponding variable on the stack
- nr_pbchsim: a new array for the SSB PDUs has been introduced

Further, these changes are now necessary:

- nr_dlsim:  there were variables "rel15" and "pdsch_pdu_rel15" that
  point to the same PDSCH PDU. At least "rel15" would not exist, as the
  corresponding pointer is now populated in phy_procedures_gNB_TX(), and
  we therefore refer to the single PDSCH PDU variable with
  "pdsch_pdu_rel15", but at the place where "rel15" used to be
  initialized.
- nr_ulsim: use nr_save_ul_tti_req() to load PDUs into the RX chain
  instead of nr_schedule_response(), which does not exist anymore.