x86: drop unused code in coreboot.c

The function setup_pcat_compatibility() is weak and implemented as empty
function in board.c hence we don't have to override that with another
empty function.

monitor_flash_len is unused, drop it.

Signed-off-by: Stefan Reinauer <reinauer@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>