Removed setting the flatfield on zoom level changes

[DominicOram]

Fixes #801

Instructions to reviewer on how to test:

1. Confirm all code to do with flat field has been removed

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 95.00%. Comparing base (228d9dc) to head (2069797).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #803      +/-   ##
==========================================
- Coverage   95.01%   95.00%   -0.01%     
==========================================
  Files         116      116              
  Lines        4753     4744       -9     
==========================================
- Hits         4516     4507       -9     
  Misses        237      237              

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[noemifrisina]

lgtm, thanks.

There seems to be a test in mx-bluesky using this:
https://github.com/DiamondLightSource/mx-bluesky/blob/41d55a417b4c272dccf64571d7c6a5e0621d895e/tests/unit_tests/hyperion/device_setup_plans/test_setup_oav.py#L31
Please make sure it's also removed.