Revert "xhci: plat: Register shutdown for xhci_plat"

Pixel 2 field testers reported that when they tried to reboot their
phones with some USB devices plugged in, the reboot would get wedged and
eventually trigger watchdog reset.  Once the Pixel kernel team found a
reliable repro case, they narrowed it down to this commit's 4.4.y
backport.  Reverting the change made the issue go away.

This reverts commit b07c12517f2aed0add8ce18146bb426b14099392.

Signed-off-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>