mount: fix automount regression

mount: fix automount regression

Tom Gundersen noticed a regression where comment=systemd.automount in
fstab no longer prevented the adding of the After=foo.mount dependency
into local-fs.target. He bisected it to commit 9ddc4a26.

It turns out that clearing the default_dependencies flag is necessary
after all, in order to avoid complementing of Wants= with After= in the
target unit. We still want to add the dependencies on quota units and
umount.target though.