Remove user and group avfs

BUG=chromium:996549
TEST=CQ-passes

Change-Id: I84e3ca23c599523fd292ace391f721161233a3f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/eclass-overlay/+/2411589
Reviewed-by: Mattias Nissler <mnissler@chromium.org>
Reviewed-by: François Degros <fdegros@chromium.org>
Tested-by: François Degros <fdegros@chromium.org>
Commit-Queue: François Degros <fdegros@chromium.org>