Sandboxes: remove duplicate button

[josephjclark]

Tiny PR to remove the unimplemented "Duplicate Sandbox" button

AI Usage

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• I have not used AI

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• I have performed an AI review of my code (we recommend using /review
  with Claude Code)
• I have implemented and tested all related authorization policies.
  (e.g., :owner, :admin, :editor, :viewer)
• I have updated the changelog.
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.2%. Comparing base (44cacfb) to head (db2f4a3).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4767   +/-   ##
=====================================
  Coverage   90.2%   90.2%           
=====================================
  Files        442     442           
  Lines      22371   22369    -2     
=====================================
+ Hits       20181   20185    +4     
+ Misses      2190    2184    -6     

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