[MRG] Reverting openmp compilation flags for macOS

[dcoeurjo]

Types of changes

Reverting the openmp flags on macOS. For some reason, the new test test/test_ot_openmp.py that simply runs an emd on multiple threads returns a segfault (macOS 26).

The new flags fix the issue.

Motivation and context / Related issue

openmp threads crash on macOs

How has this been tested (if it applies)

python test/test_ot_openmp.py

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 96.77%. Comparing base (e164e78) to head (91c759c).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #797   +/-   ##
=======================================
  Coverage   96.77%   96.77%           
=======================================
  Files         107      107           
  Lines       22342    22342           
=======================================
  Hits        21622    21622           
  Misses        720      720           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.