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.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/usb.h

@@ -1257,8 +1257,6 @@ struct usb_device_driver {
 #define	to_usb_device_driver(d) container_of(d, struct usb_device_driver, \
 		drvwrap.driver)
 
-extern struct bus_type usb_bus_type;
-
 /**
  * struct usb_class_driver - identifies a USB driver that wants to use the USB major number
  * @name: the usb class device name for this driver.  Will show up in sysfs.