cleanup: remove defaultfs validaton
cleanup: remove defaultfs validaton

According to spec, defaultFS is not MUST any more

Signed-off-by: Ma Shimiao <4511f16e942ad3568742865d09c9060c1d8bebb9@cn.fujitsu.com>

liangchenye/ocitools,opencontainers/runtime-tools,opencontainers/ocitools,opencontainers/ocitools,wking/ocitools-v2,wking/ocitools-v2,liangchenye/ocitools,opencontainers/runtime-tools