task job mgr: use broadcast_mgr.get_updated_rtconfig interface

* Spotted whilst reviewing cylc#6330
* Replace duplicated logic with a centralised implementation.

cylc/flow/task_job_mgr.py

@@ -51,10 +51,6 @@
  is_remote_platform
 )
 from cylc.flow.job_file import JobFileWriter
-from cylc.flow.parsec.util import (
- pdeepcopy,
- poverride
-)
 from cylc.flow.pathutil import get_remote_workflow_run_job_dir
 from cylc.flow.platforms import (
  get_host_from_platform,
@@ -1116,14 +1112,9 @@ def _prep_submit_task_job(
  return itask
 
  # Handle broadcasts
- overrides = self.task_events_mgr.broadcast_mgr.get_broadcast(
- itask.tokens
+ rtconfig = self.task_events_mgr.broadcast_mgr.get_updated_rtconfig(
+ itask
  )
- if overrides:
- rtconfig = pdeepcopy(itask.tdef.rtconfig)
- poverride(rtconfig, overrides, prepend=True)
- else:
- rtconfig = itask.tdef.rtconfig
 
  # BACK COMPAT: host logic
  # Determine task host or platform now, just before job submission,