remove parallel omp cython code & parallel CICD matrix items

[snowman2]

Currently not possible. Only one thread can use one PJ* at a time. https://github.com/OSGeo/PROJ/blob/f567ee4b8739505716cbb000093d78678a97b170/src/iso19111/c_api.cpp#L338

It was unstable. See #380

[djhoese]

Could you explain why you removed the nogil as part of this PR? Doesn't this mean that if I use pyproj in one thread that I'm now stuck in all other threads until pyproj is finished? I understand only using one PJ* at a time, but I think removing the nogil makes it so that nothing can be done in parallel while pyproj is working.

[snowman2]

It was out of an overabundance of caution. I wanted to make sure there was a clear understanding of how/when things should be used. Future plans to release GIL in #386

[snowman2]

Also, the gil was never released in any of the pyproj code to date, so it will be new once added.