fbcon: remove soft scrollback code

This (and the VGA soft scrollback) turns out to have various nasty small
special cases that nobody really is willing to fight.  The soft
scrollback code was really useful a few decades ago when you typically
used the console interactively as the main way to interact with the
machine, but that just isn't the case any more.

So it's not worth dragging along.

Tested-by: Yuan Ming <[email protected]>
Tested-by: Willy Tarreau <[email protected]>
Acked-by: Bartlomiej Zolnierkiewicz <[email protected]>
Acked-by: Daniel Vetter <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>