caif: remove caif_shm

caif_shm is an old implementation
caif_shm will be replaced by caif_virtio

[ As explained by Linus Walleij: "U5500 used this, but was cancelled
  and the silicon did not reach anyone outside ST-Ericsson.  Then for
  the next platforms, we have gone for the leaner & cleaner approach
  of using virtio, rpmesg and rproc." ]

Signed-off-by: Erwan Yvin <erwan.yvin@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Sjur Brendeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>