Remove deprecated partial

[BrentWheeldon]

Summary

The menu item still renders as expected in the DOM, just without the DEPRECATION WARNING: Using the "spree/admin/shared/_promotion_sub_menu" partial is deprecated, please use MenuItem#children instead. deprecation warning in the logs.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.66%. Comparing base (320e06b) to head (1f409cc).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6431   +/-   ##
=======================================
  Coverage   89.66%   89.66%           
=======================================
  Files         990      990           
  Lines       20792    20792           
=======================================
  Hits        18644    18644           
  Misses       2148     2148           

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[tvdeyen]

Thanks