refactor(changelog): removes unused code

[bhelgs]

Description

duplicate code exists in changelog_parser.py.
This bit of code seemed to be unused/untested and contributing to lower overall code coverage.

Checklist

• Add test cases to all the changes you introduce
• Run ./scripts/format and ./scripts/test locally to ensure this change passes linter check and test
• Test the changes on the local machine manually
• Update the documentation for the changes

Expected behavior

Steps to Test This Pull Request

Additional context

[codecov]

Codecov Report

Merging #541 (ed636ba) into master (53c66f6) will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #541      +/-   ##
==========================================
+ Coverage   98.01%   98.26%   +0.24%     
==========================================
  Files          39       39              
  Lines        1563     1556       -7     
==========================================
- Hits         1532     1529       -3     
+ Misses         31       27       -4     

Flag	Coverage Δ
unittests	98.26% <100.00%> (+0.24%)	⬆️

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Impacted Files	Coverage Δ
commitizen/changelog.py	99.43% <ø> (+2.16%)	⬆️
commitizen/__version__.py	100.00% <100.00%> (ø)

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