x86: acpi: Remove acpi_create_ssdt_generator()

This acpi_create_ssdt_generator() currently does nothing.
Remove this for now.

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