tty: vt, consw->con_scrolldelta cleanup

* allow NULL consw->con_scrolldelta (some consoles define an empty
  hook)
* => remove empty hooks now
* return value of consw->con_scrolldelta is never checked => make the
  function void
* document consw->con_scrolldelta a bit

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Cc: linux-usb@vger.kernel.org
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: "James E.J. Bottomley" <jejb@parisc-linux.org>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: linux-parisc@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/video/console/mdacon.c

@@ -505,11 +505,6 @@ static int mdacon_blank(struct vc_data *c, int blank, int mode_switch)
 	}
 }
 
-static int mdacon_scrolldelta(struct vc_data *c, int lines)
-{
-	return 0;
-}
-
 static void mdacon_cursor(struct vc_data *c, int mode)
 {
 	if (mode == CM_ERASE) {
@@ -578,7 +573,6 @@ static const struct consw mda_con = {
 	.con_switch =		mdacon_switch,
 	.con_blank =		mdacon_blank,
 	.con_set_palette =	mdacon_set_palette,
-	.con_scrolldelta =	mdacon_scrolldelta,
 	.con_build_attr =	mdacon_build_attr,
 	.con_invert_region =	mdacon_invert_region,
 };