*: remove the tombstone store metrics

[rleungx]

What problem does this PR solve?

Issue Number: Ref #5839.

What is changed and how does it work?

After the store becomes tombstone, the metrics still exist. We should delete them as PD does. Our integration test relies on these metrics to determine if the stores are balanced.

Check List

Tests

• Unit test

Release note

None.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• CabinfeverB
• lhy1024

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[codecov]

Codecov Report

Merging #7504 (3d28e97) into master (995fcef) will increase coverage by 0.12%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7504      +/-   ##
==========================================
+ Coverage   74.46%   74.59%   +0.12%     
==========================================
  Files         452      452              
  Lines       49942    49944       +2     
==========================================
+ Hits        37190    37254      +64     
+ Misses       9412     9368      -44     
+ Partials     3340     3322      -18     

Flag	Coverage Δ
unittests	74.59% <100.00%> (+0.12%)	⬆️

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[rleungx]

/merge

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[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 3d28e97