Retire old ADM 5120 port. It never grew much beyond the original port.
It came into the tree with the project/mips merge 8 years ago. At the
time, it was hard to find a board with enough RAM to run. Now FreeBSD
requires at least 2x the RAM it did then. No changes have happened to
this port apart from API churn and license tagging since then. It ran
OK at the time it was committed, but no sightings in the wild have
happened since shortly after it was committed.

https://www.linux-mips.org/wiki/Adm5120_devices lists a bunch of
boards that were available 5 years ago (but are no longer
available). The beefiest one had only 64MB of RAM which is too
small. The Mirktik RB1xx never had more than 32MB.

Also remove confusing QEMU config file that never ever worked in QEMU
for mips. MALTA is used for that. Another of my past mistakes, false
starts that never amounted to anything.

Discussed on: mips@ (with some dissent)