USB: remove usb_bus_type from usb.h

USB: remove usb_bus_type from usb.h

We have 2 forward declarations of usb_bus_type, one in the system-wide
usb.h and the other in the "USB core only header file".  This variable
is not exported from the USB core, so remove the declaration from usb.h
as it does not need to be there.

Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: linux-usb@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Link: https://lore.kernel.org/r/20210226102356.716746-1-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>