xhci: rip out background transfer code

xhci: rip out background transfer code

original xhci code (the one which used libusb directly) used to use
'background transfers' for iso streams.  In upstream qemu the iso
stream buffering is handled by usb-host & usb-redir, so we will
never ever need this.  It has been left in as reference, but is dead
code anyway.  Rip it out.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 331e9406f152b6bae6859a153d36e5076c58901d)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>