systemd: Use upstream tmpfiles.d rules

This fixes a failing assert in systemd-timesyncd (issue #5913) as it
expects the directory /run/systemd/netif/links/ to exist, and nothing in
NixOS currently creates it.

Also we get a net reduction in our code as rules for /run/utmp and
/var/log/journal are also provided by the same upstream file.

nixos/modules/system/activation/activation-script.nix

@@ -114,10 +114,6 @@ in
       ''
         # Various log/runtime directories.
 
-        touch /run/utmp # must exist
-        chgrp ${toString config.ids.gids.utmp} /run/utmp
-        chmod 664 /run/utmp
-
         mkdir -m 0755 -p /run/nix/current-load # for distributed builds
         mkdir -m 0700 -p /run/nix/remote-stores