fix(mf6): omit removed fields from generated code

[wpbonelli]

Filter out fields marked in DFNs as removed from the generated classes in the MF6 module. Before, removed fields were included in the generated classes and would be written to input files, which would cause MF6 to raise an error.

And update usages of inner_hclose and outer_hclose, these options have recently been removed from MF6.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.1%. Comparing base (556c088) to head (8838269).
⚠️ Report is 197 commits behind head on develop.

Files with missing lines	Patch %	Lines
flopy/mf6/utils/codegen/__init__.py	0.0%	2 Missing ⚠️
flopy/mf6/mfmodel.py	0.0%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #2804      +/-   ##
==========================================
+ Coverage     55.5%   70.1%   +14.6%     
==========================================
  Files          644     315     -329     
  Lines       124135   66066   -58069     
==========================================
- Hits         68947   46345   -22602     
+ Misses       55188   19721   -35467     

Files with missing lines	Coverage Δ
flopy/mf6/utils/codegen/filters.py	0.0% <ø> (ø)
flopy/mf6/mfmodel.py	57.3% <0.0%> (-23.6%)	⬇️
flopy/mf6/utils/codegen/__init__.py	0.0% <0.0%> (ø)

... and 591 files with indirect coverage changes

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