Remove logic for checking if zoom level is available

[shihab-dls]

Fixes #884

EPICS enum records should error if provided an unspecified value; thus, this PR removes logic checking for unspecified values in the ZoomController.

Instructions to reviewer on how to test:

1. Test if setting the str signal level to an unspecified zoom level raises an error.

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 97.50%. Comparing base (aab178f) to head (a867b55).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1018      +/-   ##
==========================================
- Coverage   97.51%   97.50%   -0.01%     
==========================================
  Files         152      152              
  Lines        6351     6342       -9     
==========================================
- Hits         6193     6184       -9     
  Misses        158      158              

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[DominicOram]

This works but the error message is obtuse (see bluesky/ophyd-async#750). If there is some reason we can't improve the error in ophyd_async then we may want to keep this. I suggest we wait for that issue to have some discussion

[DominicOram]

Looks like this will get fixed in ophyd_async (bluesky/ophyd-async#752) so happy to merge this