VTOrc: Cleanup node registration and unused code

[GuptaManan100]

Description

This PR does more cleanup in VTOrc, removing most of the code around node health and election. This code was useful for orchestrator wherein multiple nodes would use the same MySQL instance.

Other unused code and fields have been removed as well as part of this PR.

Related Issue(s)

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]

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[deepthi]

I see we removed that table. Where does this map to now? Is it helpful to document that?

[GuptaManan100]

Nothing, we don't store promotion rules in the tables anymore. They're gotten from the interface implementation.

[deepthi]

Can you explain why we don't need to store this any more? It's hard to see when you are reviewing a diff where we store the cell info which essentially provides the same functionality.

[GuptaManan100]

We still store it, we just don't need to read it at this point, because it was unused. ERS code will read it anyways when it gets all the tablets from the shard.

[codecov]

Codecov Report

Attention: Patch coverage is 37.50000% with 20 lines in your changes are missing coverage. Please review.

Project coverage is 68.13%. Comparing base (cef95ab) to head (c4cecae).
Report is 5 commits behind head on main.

Files	Patch %	Lines
go/vt/vtorc/logic/vtorc.go	0.00%	14 Missing ⚠️
go/vt/vtorc/process/health.go	75.00%	4 Missing ⚠️
go/vt/vtorc/server/api.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15617      +/-   ##
==========================================
+ Coverage   68.06%   68.13%   +0.06%     
==========================================
  Files        1561     1556       -5     
  Lines      195288   194976     -312     
==========================================
- Hits       132927   132845      -82     
+ Misses      62361    62131     -230     

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