cluster: remove tombstone nodes on pd when prune

[nexustar]

What problem does this PR solve?

• close After “ tiup cluster prune” ，the data in tombstone is removed but tiup does not clean the corresponding data in PD #1969

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation

Release notes:

NONE

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• srstack

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.20%. Comparing base (901a375) to head (ff025bd).
Report is 1 commits behind head on master.

❗ Current head ff025bd differs from pull request most recent head 3e70ac2

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HEAD has 4 uploads less than BASE

| Flag | BASE (901a375) | HEAD (ff025bd) | |------|------|------| |tiup|4|1| |cluster|7|6|

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2044      +/-   ##
==========================================
- Coverage   55.03%   49.20%   -5.83%     
==========================================
  Files         334      324      -10     
  Lines       35925    36682     +757     
==========================================
- Hits        19771    18048    -1723     
- Misses      13773    16320    +2547     
+ Partials     2381     2314      -67     

Flag	Coverage Δ
cluster	44.21% <100.00%> (-0.06%)	⬇️
playground	15.25% <0.00%> (-0.86%)	⬇️
tiup	33.50% <ø> (-0.24%)	⬇️
unittest	?

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[APPROVALNOTIFIER] This PR is NOT APPROVED

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