Remove dead code supporting old pymatgen versions

[danielhollas]

Just a small cleanup, we now require pymatgen 2024 so we can remove some old code that supported older versions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.30%. Comparing base (baefa76) to head (9fbb1b0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7066      +/-   ##
==========================================
- Coverage   79.30%   79.30%   -0.00%     
==========================================
  Files         566      566              
  Lines       43887    43876      -11     
==========================================
- Hits        34801    34791      -10     
+ Misses       9086     9085       -1     

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