Remove fetch content integration

[boulderdaze]

The purpose of the fetch content is to support cross-compiling but musica doesn't have that requirement anymore

Ideas

• Move integration tests in fortran/test/fetch_content_integration/ to fortran/test/integration/
• Remove fortran/test/fetch_content_integration/CMakeLists.txt
• Remove the fetch content integration tests from the CI workflows

[mattldawson]

I think we still need to support cross-compiling - it's the only way the JIT-compiled code (and maybe the CUDA code also?) can be used with a fortran wrapper compiled with intel compilers

[boulderdaze]

I haven't thought about JIT-compiled code. Do you agree @K20shores? If we all are on the same page, I will delete the issue.

[K20shores]

@mattldawson @boulderdaze Could we just rework the tests so that we test cross compiling directly? I just think testing fetch content makes it hard to correctly test the current branch and we don't need to be testing if cmake's stuff works

[boulderdaze]

Yeah, I agree it makes hard to test the current branch. I think we can remove the fetch content part from the current test setup and keep the cross compiling test

[boulderdaze]

We think using fetch content is simpler to support cross compiling test. Closing the issue.