Remove outdated warning about OpenMP < 3.0. Even the now-EOL Docker images for manylinux2010 had that.
Remove outdated warning about OpenMP < 3.0. Even the now-EOL Docker images for manylinux2010 had that.

Closes https://github.com/cython/cython/issues/4574

da-woods/cython,scoder/cython,da-woods/cython,da-woods/cython,cython/cython,da-woods/cython,scoder/cython,scoder/cython,cython/cython,cython/cython,cython/cython,scoder/cython