platform/chrome: cros_ec_proto: Drop cros_ec_cmd_xfer()

Since cros_ec_cmd_xfer_status() now returns Linux error codes and all
other files use that command, remove the now-unused function
cros_ec_cmd_xfer().

Signed-off-by: Prashant Malani <pmalani@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>