planbuilder: Cleanup unused logic

[dbussink]

The getVindexInformation call was returning alternative vindexes, but that was never used by callers. getUpdateVindexInformation returned it, but it's callers also never used it.

This means we can remove this, as nothing depends on it.

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• 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 65.65%. Comparing base (696fe0e) to head (c4e71e3).
Report is 84 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15415      +/-   ##
==========================================
- Coverage   67.41%   65.65%   -1.77%     
==========================================
  Files        1560     1562       +2     
  Lines      192752   194005    +1253     
==========================================
- Hits       129952   127375    -2577     
- Misses      62800    66630    +3830     

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