Revert "Detect filesystem types supported by kernel"

This reverts commit 61b80603199cd59b20af34d606a3d8b5ba40e483.

This appears to break nested Warden, which currently abuses the previous logic
to fool it into using overlayfs instead of aufs. This commit prevents that hack
(which is nice) but we probably don't want to break nested Warden at the same
time.