Remove Darwin Support about singularity

[dtrudg]

I would agree that removing Darwin support at this time is probably the way to go. We announced the end of Singularity Desktop nearly one year ago now - https://groups.google.com/a/lbl.gov/g/singularity/c/MSsOy6TkkQY/m/vrJWcbkFBQAJ - and the code is likely not in an operational state any more, as it has not received attention since before that time. It just complicates things at present.

To date there hasn't been an indication that it'll be possible to support via community contribution, and the landscape becomes trickier with each new macOS release (tightened security, notarization stuff etc.) as well as the arrival of Apple Silicon.

Most clusters are still AMD64, rather than ARM64. I'd guess this will be true for some time, meaning that a native macOS version of Singularity has less utility over time as people upgrade their machines.

Running Singularity on an M1 mac would be an interesting way to build for ARM clusters in future, but this can be accommodated via a Vagrant based approach once that is supported on these machines somehow.