move cz_conventional_commit defaults out of project wide default

[ITingHung]

Description

commit_parser in commitizen/defaults.py is only for cz_conventional_commit rather than all commitizens. This PR is to move commit_parser from commitizen/defaults.py to conventional_commits.py

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

commit_parser in commitizen/defaults.py is removed, and will be directly retrieved from ConventionalCommitsCz.

Steps to Test This Pull Request

Additional context

Related to #818

[Lee-W]

Nice refactoring. Will merge it once CI passes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.52%. Comparing base (120d514) to head (e3cfb09).
Report is 249 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1042      +/-   ##
==========================================
+ Coverage   97.33%   97.52%   +0.19%     
==========================================
  Files          42       55      +13     
  Lines        2104     2429     +325     
==========================================
+ Hits         2048     2369     +321     
- Misses         56       60       +4     

Flag	Coverage Δ
unittests	97.52% <100.00%> (+0.19%)	⬆️

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