Remove obsolete SteinerTreeInGraphs model

[isPANN]

Summary

• Remove SteinerTreeInGraphs (i64 and One), its ILP reduction, registrations, exports, examples, and dedicated tests.
• Remove its model and reduction entries from the paper.
• Keep SteinerTree and its ILP reduction unchanged. No compatibility alias is added.

The removed model accepts edge selections based on terminal reachability without enforcing a tree. SteinerTree remains the model for connected, acyclic solutions. This removes a concrete public type; core Problem, reduction, and solver interfaces are unchanged.

Validation

• Workspace tests: 6,556 passed, 2 ignored.
• Clippy: all targets and features, warnings denied.
• Paper build passed.

10 files changed; 6 additions and 930 deletions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.22%. Comparing base (b002100) to head (1c63d73).

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #1172    +/-   ##
========================================
  Coverage   96.21%   96.22%            
========================================
  Files        1074     1070     -4     
  Lines      134038   133605   -433     
========================================
- Hits       128970   128564   -406     
+ Misses       5068     5041    -27     

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