metadata/gc: remove the noop-loop for snapshot reference
metadata/gc: remove the noop-loop for snapshot reference

The noop-loop does nothing.

The containerd doesn't have any snapshotter buckets at the beginning.
If user uses specific dir as rootfs to create container, and sets
`snapshotter` key to the container, like `overlayfs` by mistake,
the gc scheduler will try to scan the snapshotter and panic.

In order to avoid this case, remove the noop-loop here.

Signed-off-by: Wei Fu <df4176d36abdc6fd0d966de5d5471522668de494@gmail.com>

crosbymichael/containerd,estesp/containerd,estesp/containerd,vdemeester/containerd,containerd/containerd,thaJeztah/containerd,dmcgowan/containerd,crosbymichael/containerd,dmcgowan/containerd,containerd/containerd,vdemeester/containerd,thaJeztah/containerd,mikebrow/containerd,mikebrow/containerd