release: Remove remnants of pc98 support

The cons25w line was added in c991a6474749 for pc98, along with reading
MACHINE in order to determine what terminal type to use for VTYs. Commit
2b375b4edd1b removed this condition, leaving it as always using xterm
for VTYs, but left behind the now-unused MACHINE variable and the
cons25w list entry. Clean these up to be how they were before pc98
support was added.

Note that anyone who really needs a cons25w terminal can still request
it, it's just not listed as a common option.

Reviewed by: imp, brooks
Differential Revision: https://reviews.freebsd.org/D36587