Remove unused StructuredInputData

[clmould]

Description

The StructuredInputData is not used anywhere in PROCESS, and not used in bluemira core or step-bluemira either, so it should be removed

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 49.89%. Comparing base (a6cc0e2) to head (fdd6194).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4566      +/-   ##
==========================================
+ Coverage   49.87%   49.89%   +0.01%     
==========================================
  Files         151      151              
  Lines       29873    29843      -30     
==========================================
- Hits        14899    14890       -9     
+ Misses      14974    14953      -21     

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[timothy-nunn]

@ukaea/process-model-review (@je-cook @josieapeters from bluemira too) I'm going to leave this PR in draft for a few days to give one last opportunity for anyone to let us know they use this class before its gone permanently.