greybus: kill greybus_{get,put}_gbuf()

These functions are never used, so we can get rid of them.
Since there's no reference-getting function any more, we no
longer need "gbuf_mutex" to avoid racing gets and puts, so
get rid of that too.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>