refactor: remove unused code for pruning row groups

[zhongzc]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

prune_row_groups and prune_primary_key are unused.

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.

Refer to a related PR or issue link (optional)

[zhongzc]

@v0y4g3r @evenyag PTAL

[v0y4g3r]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (bad8918) 85.85% compared to head (dd0377b) 85.32%.
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2973      +/-   ##
===========================================
- Coverage    85.85%   85.32%   -0.54%     
===========================================
  Files          775      777       +2     
  Lines       124707   124919     +212     
===========================================
- Hits        107072   106585     -487     
- Misses       17635    18334     +699