Deprecate PostgreSQL usage connector

[mars-lan]

🤔 Why?

It's no longer in use as the usage is directly derived from the query log.

🤓 What?

As titled. Also cleaned up tests & updated README.

🧪 Tested?

N/A

☑️ Checks

• My PR contains actual code changes, and I have updated the version number in pyproject.toml.

[github-actions]

☂️ Python Coverage

current status: ✅

Overall Coverage

Lines	Covered	Coverage	Threshold	Status
13194	11777	89%	85%	🟢

New Files

No new covered files...

Modified Files

File	Coverage	Status
metaphor/postgresql/usage/extractor.py	0%	🟢
TOTAL	0%	🟢

updated for commit: c1af938 by action🐍

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.

Project coverage is 89.26%. Comparing base (bc733e2) to head (c1af938).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
metaphor/postgresql/usage/extractor.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1023      +/-   ##
==========================================
+ Coverage   89.24%   89.26%   +0.01%     
==========================================
  Files         202      202              
  Lines       13220    13194      -26     
==========================================
- Hits        11798    11777      -21     
+ Misses       1422     1417       -5     

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[usefulalgorithm]

Otherwise LGTM