ppc4xx: Remove sc3 board

As this board seems to be unmaintained for quite some time, and its
not moved to the generic board ingrastructure, lets remove it.

This will also enable us to remove the CONFIG_AUTOBOOT_DELAY_STR2
and CONFIG_AUTOBOOT_STOP_STR2 macros, as this sc3 board is the
only one using one of this macros. A removal patch will follow
soon.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Juergen Beisert <jbeisert@eurodsn.de>
Acked-by: Heiko Schocher <hs@denx.de>