remove unnecessary 'dielectric' field

[tsmathis]

dielectric was added as a field to Calculation in 8b23a06

frequency_dependent_dielectric is the appropriate field for this, so dielectric was a duplicate

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.19%. Comparing base (a294022) to head (49fc20f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1173      +/-   ##
==========================================
- Coverage   90.19%   90.19%   -0.01%     
==========================================
  Files         147      147              
  Lines       14508    14507       -1     
==========================================
- Hits        13085    13084       -1     
  Misses       1423     1423              

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