Remove unused subplans and improve docs

[DominicOram]

Fixes #874

Removes quite a few plan name constants where they weren't actually being used and documents the ones that are left

Instructions to reviewer on how to test:

1. Confirm tests still pass and documentation makes sense

Checks for reviewer

• Would the PR title make sense to a user on a set of release notes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.45%. Comparing base (162fc3f) to head (8414d2a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1548      +/-   ##
==========================================
- Coverage   92.46%   92.45%   -0.01%     
==========================================
  Files         146      146              
  Lines        8276     8267       -9     
==========================================
- Hits         7652     7643       -9     
  Misses        624      624              

Components	Coverage Δ
i24 SSX	78.56% <ø> (ø)
hyperion	98.18% <ø> (-0.01%)	⬇️
other	97.99% <100.00%> (-0.01%)	⬇️

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