Remove unused xeus-cling and xtensor images

[mcbarton]

As far as I can tell these images are not being used anywhere in the repo, so this PR removes them.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.94%. Comparing base (9a4af34) to head (cb45bf3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #404      +/-   ##
==========================================
- Coverage   82.29%   81.94%   -0.36%     
==========================================
  Files          21       21              
  Lines         853      853              
  Branches       87       87              
==========================================
- Hits          702      699       -3     
- Misses        151      154       +3     

see 1 file with indirect coverage changes
see 1 file with indirect coverage changes

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