mds: remove superfluous warning of releasing lease

When receiving the lease release message, it's possible that the lease
has already expired and the corresponding dentry has been trimmed from
the cache.

Fixes #5871

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
Merge pull request #872 from ceph/wip-5871

mds: remove superfluous warning of releasing lease

Reviewed-by: Sage Weil <sage@inktank.com>