usb: remove reference to deleted config STB03xxx

usb: remove reference to deleted config STB03xxx

Commit 6848317fe834 ("powerpc/40x: Remove STB03xxx") removes the config
STB03xxx, but left a reference in ./drivers/usb/Kconfig behind.

Hence, ./scripts/checkkconfigsymbols.py warns:

STB03xxx
Referencing files: drivers/usb/Kconfig

Remove this reference to the deleted config.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Link: https://lore.kernel.org/r/20210818071137.22711-4-lukas.bulwahn@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>