Remove extra MacOS runners

[spencer-lunarg]

We (KhronosGroup) have a very limited number of CI Github Action Minutes that we all need to share, Github FINALLY added a real way to view this information

https://github.com/KhronosGroup/KTX-Software/actions/metrics/usage

If you are not aware, MacOS runners are counted as 10x the minutes when used. @MarkCallow is there a real practical reason we need 16 combinations of MacOS runs per run????

For the sake of our other KhronosGroup repos, could we please reduce/limit macos.yml to only what is really needed

I highly doubt you are finding issues where it only fails on 1 of the 16, and not the other 15, other repos have also pruned their MacOS builds to what is only necessary

Thanks

[spencer-lunarg]

I bring this up as we (Vulkan Validation Layers) are constantly getting blocked our CI running and we have worked really hard slimming things down in our Github CI and even have work offloaded to internal CI runners