Fix Join Predicate Cleanup Bug in Route Merging

[systay]

Description

This PR addresses a bug in Vitess where, upon successfully merging three tables into a single Route, the join predicates were sometimes not cleaned up correctly. This issue affects all supported versions of Vitess. Given the minimal nature of the fix, it is recommended to backport this fix to all affected versions.

Related Issue(s)

Fixes #16387

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 68.68%. Comparing base (fd6411d) to head (b8a018a).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16386      +/-   ##
==========================================
- Coverage   68.68%   68.68%   -0.01%     
==========================================
  Files        1548     1548              
  Lines      199063   199084      +21     
==========================================
+ Hits       136721   136735      +14     
- Misses      62342    62349       +7     

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[frouioui]

We should add a vitess-tester test for this, otherwise looks good to me.