Revert "feat: use Geant4TVUserParticleHandler by default (#44)" (#64)

### Briefly, what does this PR introduce? Please link to any relevant
presentations or discussions.
This reverts commit 6c595b9. It has too
large implications in eic/containers#257 and
needs additional pieces..

### What is the urgency of this PR?
- [ ] High (please describe reason below)
- [x] Medium
- [ ] Low

### What kind of change does this PR introduce?
- [x] Bug fix (issue: stuck on this in
eic/containers#257)
- [ ] New feature (issue #__)
- [ ] Optimization (issue #__)
- [ ] Updated documentation
- [ ] other: __

### Please check if any of the following apply
- [ ] This PR introduces breaking changes. Please describe changes users
need to make below.
- [ ] This PR changes default behavior. Please describe changes below.
- [ ] AI was used in preparing this PR. Please describe usage below.

Author: wdconinc

src/dd4pod/python/npsim.py

@@ -44,9 +44,6 @@ def setupCerenkov(kernel):
     return None
   RUNNER.physics.setupUserPhysics(setupCerenkov)
 
-  # Define tracking region using tracking_volume defined in the parallel world of the geometry
-  RUNNER.part.userParticleHandler = "Geant4TVUserParticleHandler"
-
   # Disable warnings for unstable resonances with off-shell mass
   if hasattr(RUNNER.physics, "ESeverity"):
     RUNNER.physics.ESeverity = "IgnoreTheIssue"