Remove unused code in discovery queue creation

[GuptaManan100]

Description

This PR removes some unused code in discovery queue creation in VTOrc. It also cleans up the map and the mutex since we don't really need it, we only create a single queue in VTOrc.

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]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 67.69%. Comparing base (ec29bdb) to head (740a9f7).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtorc/discovery/queue.go	0.00%	2 Missing ⚠️
go/vt/vtorc/logic/vtorc.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17515      +/-   ##
==========================================
- Coverage   67.71%   67.69%   -0.02%     
==========================================
  Files        1584     1584              
  Lines      254519   254487      -32     
==========================================
- Hits       172338   172284      -54     
- Misses      82181    82203      +22     

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