geoipupdate: use head block
libmaxminddb: use head block
unfs3: use head block
Remove explicit xz deps from formulae with HEAD specs

These deps are only needed for the stable download, but they will get
pulled in for HEAD builds unless the are scoped appropriately (i.e.
inside a stable block).

Since they are also auto-detected from the URL now, we can just remove
them.