Remove unneeded get_cb_solution and associated cache

[odow]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.61%. Comparing base (405a031) to head (6702743).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #324      +/-   ##
==========================================
- Coverage   98.61%   98.61%   -0.01%     
==========================================
  Files           3        3              
  Lines        2524     2521       -3     
==========================================
- Hits         2489     2486       -3     
  Misses         35       35              

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[odow]

This is something that Julia does well, and we do badly: document what version that API was added.

Xpress doesn't tell me either: https://www.fico.com/fico-xpress-optimization/docs/latest/solver/optimizer/HTML/XPRSgetcallbacksolution.html