Tidy up data_structure_dicts.py

[clmould]

Description

Had to add a 2nd method of getting the DICT_DESCRIPTIONS in #4215 due to having a mix of old data structure files and new ones (with the dataclasses). Now that we only have dataclasses, the old method is no longer necessary so needs to be removed.
Also general tidying of the file, removing old, unused functions.

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.86%. Comparing base (3fe7140) to head (699a6ab).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4378      +/-   ##
==========================================
+ Coverage   48.79%   48.86%   +0.06%     
==========================================
  Files         151      151              
  Lines       29408    29351      -57     
==========================================
- Hits        14351    14341      -10     
+ Misses      15057    15010      -47     

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