Remove sandbox infrastructure directory

[lukeheath]

These are unused and triggering several dozen code security warnings. Do y'all see any issue with removing these files?

[zwass]

Definitely most interested in Robert's thoughts, but seems to me if we ever need any of these things again we will grab them from the git history.

[rfairburn]

I'm ok with doing this from my end. We can always pull it back from an old revision of the repo if we need it again.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7e3c0ef) 65.92% compared to head (af3d17a) 66.03%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16902      +/-   ##
==========================================
+ Coverage   65.92%   66.03%   +0.10%     
==========================================
  Files        1144     1144              
  Lines      101502   101537      +35     
  Branches     2530     2530              
==========================================
+ Hits        66914    67047     +133     
+ Misses      29604    29520      -84     
+ Partials     4984     4970      -14     

Flag	Coverage Δ
backend	67.13% <ø> (+0.11%)	⬆️

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