init: usb: delete rndis func to unregister n/w interface

Rndis n/w interface "usb0" registration/deregistration is broken. If a
user try to switch to other functions or disable usb tethering or unplug
the usb cable then it doesn't kill "usb0" interface.

Fix is to delete Rndis function to unregister tethering interface when
switching from tethering to other functions or disable/unplug the
tethering cable. If we don't do that then the rndis netdev interface
will never be freed or unregistered.

This also means the rndis function has to be created everytime user
enable tethering function from Settings instead of creating it at
"on boot" in init.$hardware.usb.rc like we do currently. A relevant fix,
Change-Id: Icb49020d624fb21ef2607d473948cbbf3b9cc469, for reference has
already been submitted for device/linaro/hikey

Change-Id: If1f922e02277cccdc8c0b263be63989ee102cc80
Reported-by: Winter Wang <wente.wang@nxp.com>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>