files: drop gentoo-keys

Last touched in 2018 in d73aadb but we
changed our OpenPGP infrastructure quite a while ago in an update to GLEP 63
(https://www.gentoo.org/glep/glep-0063.html#changes).

These files are out of date, unused, and not relevant to our modern-day
pgp setup. The gkeys packages have already been removed from ::gentoo
and keeping this around is likely to confuse.

Signed-off-by: Sam James <[email protected]>