gnat10, gnat9, gnat6: drop the packages

[vcunat]

They haven't succeeded for half a year and people don't seem to miss them, so let's reduce the clutter on Hydra, etc. https://hydra.nixos.org/job/nixpkgs/trunk/gnat10.x86_64-linux/all

(the usual checklist doesn't really apply here)

[tlaurion]

@vcunat actually there is a need to have gnat6 to bootstrap coreboot 4.11 buildstack, otherwise kgpe-d16 stuck to coreboot 4.11 won't ever have reproducible builds.

What are the steps to revive gnat6?
@Mic92 @ofborg @vcunat ?!

linuxboot/heads#1269 (comment)

[vcunat]

I didn't change much. They did not build. If you make it build, I see no problem with merging that.