This's an optimization to get rid of redundant `/dev/shm" mounts for pause container.
In `oci.defaultMounts`, there is a default `/dev/shm` mount which is redundant for
pause container.
Fixes: #6911
Signed-off-by: Jiang Liu <gerry@linux.alibaba.com>
Signed-off-by: Lei Wang <wllenyj@linux.alibaba.com>