refine MAC Debug output

- uncomment "Couldn't find RNTI for UE X" in UE_RNTI() and rb_alloc in
  pre_processor_reset() as they only inflate the debug output with little
  benefited
- "doing ue_schedule_spec" is now displayed for individual UEs (and individual
  CCs)
- show messages when not scheduling UE in ue_schedule_spec() due to invalid
  slice or if not in RRC_CONNECTED