remove _libgcc_mutex

[h-vetinari]

After conda-forge/ctng-compilers-feedstock#167, this is the last place in conda-forge that depends on the old _libgcc_mutex, which was done (AFAIU) for the libgcc -> libgcc-ng split. Many year later, this has since been resolved (back to libgcc) with conda-forge/ctng-compilers-feedstock#148, which makes them equivalent (libgcc-ng is now a compat wrapper around libgcc), and we've removed remaining usages of libgcc-ng in conda-forge.

Closes conda-forge/_libgcc_mutex-feedstock#6

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.

[h-vetinari]

I couldn't get cross-compilation to work, it fails with

+ ldd ./omp_hello
	not a dynamic executable

even though we've just successfully compiled & executed omp_hello the line before, and even when specifying a fully qualified path to ldd in the proper sysroot. I don't understand what's going wrong there...

[beckermr]

That's super weird but emulated builds are fine. LGTM!