Remove legacy EmergencyReparentShard stats

[timvaillancourt]

Description

This PR removes legacy ERS stats that were deprecated by a PR I wrote in Vitess 18.0. Replacement metrics with labels were added in the same release

Related Issue(s)

#13723

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
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Tests

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Backward compatibility

• Protobuf changes should be wire-compatible.
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• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.41%. Comparing base (f118ba2) to head (98dc9b8).
Report is 93 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15941      +/-   ##
==========================================
+ Coverage   68.40%   68.41%   +0.01%     
==========================================
  Files        1556     1562       +6     
  Lines      195121   197053    +1932     
==========================================
+ Hits       133479   134821    +1342     
- Misses      61642    62232     +590     

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[deepthi]

👍