Improving FreeBSD support

I added FreeBSD support to GHCUP, but I have come around to the view that we should remove it, as GHC FreeBSD binaries are not published quickly or reliably enough for GHCUP's purposes - it's holding back our "recommended version" from advancing, isn't it?

Getting FreeBSD back into ghcup should wait until FreeBSD is a first-class-citizen of GHC again.