core: mount namespaces: Remove auxiliary bind mounts directory after unit termination

core: mount namespaces: Remove auxiliary bind mounts directory after unit termination

Unit that requires its own mount namespace creates a temporary directory
to implement dynamic bind mounts (org.freedesktop.systemd1.Manager.BindMountUnit).
However, this directory is never removed and they will accumulate for
each unique unit (e.g. templated units of systemd-coredump@).

Attach the auxiliary runtime directory existence to lifetime of other
"runtime" only per-unit directories.