x86: Remove CONFIG_USB_MAX_CONTROLLER_COUNT

As all x86 boards have been switched over to use DM USB, remove
CONFIG_USB_MAX_CONTROLLER_COUNT which is not used by DM USB.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Stefan Roese <sr@denx.de>