x86: coreboot: Drop USB init on startup

This is very annoying as it is quite slow on many machines. Also, U-Boot
has an existing 'preboot' mechanism to enable this feature if desired.

Drop this code so that it is possible to choose whether to init USB or
not.

Use the existing USE_PREBOOT mechanism instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>