Remove deprecated --disable_active_reparents flag

[GuptaManan100]

Description

This PR deletes the flag deprecated in #14871.

Related Issue(s)

#17734

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]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.55%. Comparing base (74d9949) to head (dec9655).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17919      +/-   ##
==========================================
- Coverage   67.56%   67.55%   -0.01%     
==========================================
  Files        1597     1597              
  Lines      259775   259757      -18     
==========================================
- Hits       175509   175490      -19     
- Misses      84266    84267       +1     

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[deepthi]

Don't we need a note in the release notes summary for this?

[deepthi]

Test failures are probably because tests are still using the flag.