Remove incorrect use of GrowFactors() in Policy and Parameters classes

[martinholmer]

Fixes #2851 by removing two self._gfactors = GrowFactors() statements and making associated code changes.

Also, simplifies Policy.set_rates logic by eliminating code that has been obsolete since April 2024 when PR #2732 was merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.42%. Comparing base (0411f9c) to head (f71b8b1).
Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2852      +/-   ##
==========================================
- Coverage   99.42%   99.42%   -0.01%     
==========================================
  Files          13       13              
  Lines        2619     2618       -1     
==========================================
- Hits         2604     2603       -1     
  Misses         15       15              

Flag	Coverage Δ
unittests	99.42% <100.00%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
taxcalc/__init__.py	100.00% <100.00%> (ø)
taxcalc/calculator.py	100.00% <100.00%> (ø)
taxcalc/parameters.py	100.00% <100.00%> (ø)
taxcalc/policy.py	100.00% <100.00%> (ø)
taxcalc/taxcalcio.py	100.00% <100.00%> (ø)