check Msg2 allocation frame/slot in DL scheduler

Move decision of Msg2 allocation time instant from instance we receive
RA indication (only containing preamble time information) to actual RA
scheduler (containing current DL slot time). This has the advantage:

- if Msg2 cannot be sent, we might try in a later slot
- no timing-based problems were a RACH.indication FAPI message might
  come late, we calculate a Msg2 frame/slot, but this instant has
  already passed in the DL scheduler
- no assumption about slot_ahead (an L1 constant)

The previous code calculated and verified an SSB index; I am not sure
this code is correct or used. I leave the function
ssb_index_from_prach() (hence the gcc attribute), but remove any checks.
Similarly, the beam association list might be wrong. I deleted the
corresponding code.