usb: remove old usb-host code

usb: remove old usb-host code

The usb-host code has been rewritten for qemu 1.5 to use libusb,
the old code has been left in as temporary fallback.  Now we are
two releases further out, targeting the 1.7 release.  No major
issues with the new code poped up until now.  Time to remove it
from tre tree.  Should we ever need it again for some reason --
git has a copy for us in the history.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>