usb: host: remove ehci-pxa168.c

Since ehci-mv.c can cover Marvell PXA and MMP series including PXA168,
so this driver seems redundant now.

Signed-off-by: Neil Zhang <zhangwm@marvell.com>
Cc: <tanmay.upadhyay@einfochips.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>