metrics/cgroups/v1: Remove unused event parameter
metrics/cgroups/v1: Remove unused event parameter

The event parameter wasn't actually used when processing oom events,
likely because it's only ever available for reads.

Additionally clarify flush is for eventfds, and point to where the
buffer size of 8 is coming from.

Signed-off-by: Daniel Canter <8d0fd95ae71f7e4870b010b10f83a6201b034a65@microsoft.com>

estesp/containerd,mikebrow/containerd,estesp/containerd,thaJeztah/containerd,containerd/containerd,mikebrow/containerd,containerd/containerd,thaJeztah/containerd