Remove OpenMP support from EMD

[nathanneike]

Types of changes

• Refactoring / code cleanup
• Breaking change

Motivation and context / Related issue

This PR removes the OpenMP-specific EMD implementation and the related build-time OpenMP detection. The threaded backend was not providing enough value to justify the extra solver path, build complexity, and platform-specific maintenance burden.

numThreads is kept temporarily as a compatibility argument, but it now falls back to the single-threaded solver and emits a warning when set to a value greater than 1.

How has this been tested (if it applies)

• Ran the full test suite:
  • .venv/bin/python -m pytest --durations=20 -v test/ --doctest-modules --ignore ot/gpu/
• Result:
  • 2490 passed, 424 skipped, 6 xfailed

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.80%. Comparing base (41a4d57) to head (36cb8dd).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #805      +/-   ##
==========================================
- Coverage   96.87%   96.80%   -0.08%     
==========================================
  Files         113      112       -1     
  Lines       23062    23045      -17     
==========================================
- Hits        22342    22309      -33     
- Misses        720      736      +16     

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