shared/but-util: drop trusted annotation from bus_open_system_watch_bind_with_description()

      https://bugzilla.redhat.com/show_bug.cgi?id=1746057

This only affects systemd-resolved. bus_open_system_watch_bind_with_description()
is also used in timesyncd, but it has no methods, only read-only properties, and
in networkd, but it annotates all methods with SD_BUS_VTABLE_UNPRIVILEGED and does
polkit checks.