refactor(BaseCommitizen): remove NotImplementedError and make them ab…

[bearomorphism]

…stract method

Description

• Removed the NotImplementedErrors in the abstract class BaseCommitizen
• Removed the test file which tests the behavior of BaseCommitizen. I believe we don't need to test the abstract class.

I have read the git blame to those changes. I didn't find a reason to not make them abstract.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (v4-9-2@cc981fc). Learn more about missing BASE report.

Additional details and impacted files

@@            Coverage Diff            @@
##             v4-9-2    #1608   +/-   ##
=========================================
  Coverage          ?   98.25%           
=========================================
  Files             ?       58           
  Lines             ?     2698           
  Branches          ?        0           
=========================================
  Hits              ?     2651           
  Misses            ?       47           
  Partials          ?        0           

Flag	Coverage Δ
unittests	98.25% <100.00%> (?)

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