net: mhi: Remove MBIM protocol

The MBIM protocol has now been integrated in a proper WWAN driver. We
can then revert back to a simpler driver for mhi_net, which is used
for raw IP or QMAP protocol (via rmnet link).

- Remove protocol management
- Remove WWAN framework usage (only valid for mbim)
- Remove net/mhi directory for simpler mhi_net.c file

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>