And in particular make it not fail when removing an already removed
sandbox pod. According to the CRI spec:
[RemovePodSandbox] is idempotent, and must not return an error if
the sandbox has already been removed.
We now only print a warning instead of returning an error.
We still return an error when the passed pod ID is empty.
Fixes#240
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>