Move beamstop to use an XYZPositioner and remove old device

[DominicOram]

Fixes #685

Instructions to reviewer on how to test:

1. Confirm tests still pass
2. Confirm dodal connect i04 passes

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards
• If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
• Have the connection tests for the relevant beamline(s) been run via dodal connect ${BEAMLINE}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.46%. Comparing base (1ff9784) to head (2372f40).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #695      +/-   ##
==========================================
- Coverage   94.46%   94.46%   -0.01%     
==========================================
  Files         116      115       -1     
  Lines        4611     4604       -7     
==========================================
- Hits         4356     4349       -7     
  Misses        255      255              

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