- don't call exit() if get_message() fails, simply terminate the thread and
  disable the T tracer
- T_dont_fork disappears, monitor_and_kill() as well, so now it's only 2
  processes: the main (enb/gnb/ue) and the local tracer

Removes phy_procedures_gNB_common_RX() (only used in nr_ulsim) + fixes
in pucchsim