tmpfiles: don't do automatic cleanup in $XDG_RUNTIME_DIR

Now that logind will clean up all IPC resources of a user we should
really consider $XDG_RUNTIME_DIR as just another kind of IPC with the
same life-cycle logic as the other IPC resources. This should be safe
now to do since every user gets his own $XDG_RUNTIME_DIR tmpfs instance
with a fixed size limit, so that flooding of it will more effectively be
averted.