fix: remove keyspace when merging subqueries

[systay]

Description

When the planner is able to merge subqueries, it's important to remember to remove the keyspace from the query before adding the subquery to the outer query.

Related Issue(s)

Fixes #14864

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
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• Documentation was added or is not required

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.23%. Comparing base (8f3c035) to head (0e97acd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16019      +/-   ##
==========================================
- Coverage   68.25%   68.23%   -0.02%     
==========================================
  Files        1541     1541              
  Lines      197103   197104       +1     
==========================================
- Hits       134535   134497      -38     
- Misses      62568    62607      +39     

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