acorn26: econet

[Maxime Villard <max%m00nbsd.net@localhost>]

Hi,
following the removal of port-acorn26 two weeks ago, we are left with no
kernel user of the Econet code.

The only users were in acorn26/ioc/if_eca.c, and they're now gone. There are
no good reasons to keep the code anymore, it should go to the attic like
acorn26 did, since it was acorn26-specific.

Most of the code is in net/if_eco.h and net/if_ecosubr.c, with very few
references outside.

Ok?

Maxime