usb: gadget: Remove unused usb_remove_config

usb: gadget: Remove unused usb_remove_config

usb_remove_config() was added in 2012's
commit Fixes: 51cce6fc155c ("usb: gadget: composite: Add
usb_remove_config")
but has remained unused.

I see there was a use in drivers/staging/cch that
was removed by
commit 515e6dd20b3f ("Staging: ccg: delete it from the tree")
but it had it's own copy of usb_remove_config()

Remove it.

Signed-off-by: "Dr. David Alan Gilbert" <linux@treblig.org>
Link: https://lore.kernel.org/r/20250608233338.179894-3-linux@treblig.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>