vtorc: remove duplicate instance read from backend

[timvaillancourt]

Description

This PR prevents a duplicate read of an instance in inst.ReadTopologyInstanceBufferable(...), which is a pretty hot-path func

Immediately before fullStatus(...) is called we call inst.ReadTablet(...) and today fullStatus(...) immediately does the same inst.ReadTablet(...) read again

Related Issue(s)

#17330

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

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.46%. Comparing base (2118bc3) to head (45b5882).

Files with missing lines	Patch %	Lines
go/vt/vtorc/inst/instance_dao.go	0.00%	1 Missing ⚠️
go/vt/vtorc/inst/tablet_dao.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17834      +/-   ##
==========================================
+ Coverage   67.45%   67.46%   +0.01%     
==========================================
  Files        1592     1592              
  Lines      258167   258163       -4     
==========================================
+ Hits       174143   174172      +29     
+ Misses      84024    83991      -33     

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