autoboot.c: Remove CONFIG_AUTOBOOT_STOP_STR2 and CONFIG_AUTOBOOT_DELAY_STR2

These defines for a 2nd autoboot stop and delay string are nearly unused. Only
sc3 defines CONFIG_AUTOBOOT_DELAY_STR2. And a patch to remove this most likely
unmaintained board is also posted to the list.

By removing these defines the code will become cleaner and moving the remaining
compile options to Kconfig will get easier.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Heiko Schocher <hs@denx.de>