tidy: Remove unused BackwardsBeamPipe_geo (#991)

### Briefly, what does this PR introduce?
Remove the old BackwardsBeamPipe_geo file as it is currently unused.
Current Beampipe geometry is now implemented using BeamPipeChain_geo
which can handle both individual beampipe elements and correctly joining
them together.

### What kind of change does this PR introduce?
- [ ] Bug fix (issue #__)
- [ ] New feature (issue #__)
- [ ] Documentation update
- [x] Other: Tidy up

### Please check if this PR fulfills the following:
- [ ] Tests for the changes have been added
- [ ] Documentation has been added / updated
- [ ] Changes have been communicated to collaborators

### Does this PR introduce breaking changes? What changes might users
need to make to their code?
No
### Does this PR change default behavior?
No

Author: simonge