postinst: drop transitional systemd watchdog mux socket cleanup

postinst: drop transitional systemd watchdog mux socket cleanup

This transitional code was added first with
commit f8a3fc80af299e613c21c9b67e29aee8cc807018
and fixed up with
commit ecc145c9724f056549e5458f17d7714ac8c83459
during Proxmox VE 4.1 and 4.2 to remove the problematic systemd
managed watchdog mux socket.

As each system going for an distribution upgrade must first upgrade
to 4.4, where this gets handled, we can remove it now.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>