Garbage collect the a83t emac overlays

The 4.16 DTS import brought in emac support for the a83t. Since these
boards' DTS is pulled from /boot and I forgot to hook these up to the build,
they should be fairly safe to go away.

The a83t-sid and h3-sid overlays are still relevant. a83t-sid will likely
come in with 4.18 DTS.