Remove unused code for old START_GTID logic

[dbussink]

This was disabled in #16424, but we can clean up this additional code since it's unused.

Related Issue(s)

Follow up to #16424

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

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.38%. Comparing base (c5d0ecc) to head (72a2a8c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17265   +/-   ##
=======================================
  Coverage   67.37%   67.38%           
=======================================
  Files        1573     1573           
  Lines      253110   253085   -25     
=======================================
- Hits       170535   170531    -4     
+ Misses      82575    82554   -21     

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