autobuild: Drop py2 autobuild jobs

autobuild: Drop py2 autobuild jobs

Samba v4.11 will no longer support python2, so let's drop the autobuild
jobs. This will save some gitlab/sn-devel time and money, as it's less
work for CI to do.

Note that this highlights some previous inconsistencies:
- samba-none-env-py2 was being built for gitlab but not sn-devel.
- samba-nt4-py2 was being built for sn-devel but not gitlab

I've left samba-buildpy2-only for now, which will be addressed in a
subsequent patch.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>