Minor MIPS tree pruning

[Warner Losh]

I'd like to propose that we eliminate support for the following before the
FreeBSD 12 branch.

adm5120 (most boards don't have enough memory, very old)
alchemy (the au1xxxx port never really was finished, and boards lack
memory, very old)
idt (only a few boards worked, most are long obsolete)
rt305x (raylink kit is now out dated, at least I think so)
sibyte (long obsolete, hard to get hardware)

I thought I'd post a heads up here before I proposed this list to arch at .

We have enough exemplars these days we don't need to keep these older ports
around as examples anymore, and I have my doubts if we even work on these
boards anymore.

If I'm wrong about these being old, or that FreeBSD is no longer running on
them, please let me know. Thanks!

Warner