fbdev: Garbage collect fbdev scrolling acceleration, part 1 (from TODO list)

Scroll acceleration is disabled in fbcon by hard-wiring
p->scrollmode = SCROLL_REDRAW. Remove the obsolete code in fbcon.c
and fbdev/core/

Signed-off-by: Claudio Suarez <cssk@net-c.es>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/YVXTYqszZix9TxjJ@gineta.localdomain