sparc: Kill PROM console driver.

Many years ago when this driver was written, it had a use, but these
days it's nothing but trouble and distributions should not enable it
in any situation.

Pretty much every console device a sparc machine could see has a
bonafide real driver, making the PROM console hack unnecessary.

If any new device shows up, we should write a driver instead of
depending upon this crutch to save us.  We've been able to take care
of this even when no chip documentation exists (sunxvr500, sunxvr2500)
so there are no excuses.

Signed-off-by: David S. Miller <davem@davemloft.net>