Remove unused dependency OpenMP

[siggmo]

While working on #25, I stumbled across the OpenMP dependency in the CMakeLists.txt file. It was not covered by our listed dependencies when I tried to install them with brew on macOS. So I wanted to check if it was even needed, and it actually isn't. Building FANS as well as all test cases succeed without OpenMP. I guess I confused it with openmpi in the first place, since I'm not really familiar with both.

This PR removes OpenMP as a dependency in the CMake recipe.

[IshaanDesai]

I think @sanathkeshav has some plans to include shared memory parallelization using OpenMP, and hence he probably kept the dependency.

[sanathkeshav]

Hi Moritz!

Thanks for the PR. You are absolutely right that OpenMP is currently not used at all.
But I think you included it in the CMake recipe because I had included it in the old recipe.

As @IshaanDesai mentioned, Yes I plan to use it in the near future so we should leave it as is at the moment.

For some extra context: FFTW allows for MPI parallelism via slab domain decomposition only. So within a slab, we would like to introduce shared memory parallelism via OpenMP.

[siggmo]

Okay then let's add it to the installation instructions as well, right? At least on the macOs runner it was not preinstalled and thus the CMake recipe failed.

[IshaanDesai]

Okay then let's add it to the installation instructions as well, right? At least on the macOs runner it was not preinstalled and thus the CMake recipe failed.

We can add this directly on develop.