xfree86: drop unused xf86SetReallySlowBcopy()

This had been introduced almost two decades ago, by Dave Airlie (*1) along with some major IO speed improvement, just in case some driver still needed the old behaviour - in that case it would call xf86SetReallySlowBcopy(), so xf86SlowBcopy() would fall back to the old approach emitting an extra outb() on debug port, in order to slow things down more.

Now aeons have passed and there doesn't seem to be any actual user for this, so it's time to drop that ancient relic.

*1) commit e717eb82

Signed-off-by: Enrico Weigelt, metux IT consult info@metux.net