4400 remove hardcoded bc20m and tc0m

[chris-ashe]

Description

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

❌ Patch coverage is 36.06557% with 39 lines in your changes missing coverage. Please review.
✅ Project coverage is 49.13%. Comparing base (6a9a047) to head (8da890b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/stellarator/coils/coils.py	4.54%	21 Missing ⚠️
process/models/pfcoil.py	42.85%	8 Missing ⚠️
process/models/tfcoil/superconducting.py	20.00%	8 Missing ⚠️
process/models/superconductors.py	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4478      +/-   ##
==========================================
- Coverage   49.14%   49.13%   -0.01%     
==========================================
  Files         152      152              
  Lines       29814    29821       +7     
==========================================
+ Hits        14652    14654       +2     
- Misses      15162    15167       +5     

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[clmould]

A few small things

[clmould]

1 tiny thing