gypsum.pike: Drop the 8.0 markers from the compat mapping.

There's no point having those and the corresponding 8.1 markers; if a version
isn't listed, it's presumed to have been bug-free since it was cut. I've never
actually keyed in 7.9 markers, so I'm just going to hope that everybody on
7.9.x upgrades to 8.0 or 8.1 - there's not a lot of point trying to support
an old dev version.