core/serialization: drop misleadingly-named unit_can_serialize()

All unit types can be serialized. This function was really checking whether the
unit type has custom serialization/deserialization code. But we don't need a
function for this.

Also, the check that both .serialize() and .deserialize_item() are defined is
better written as an assert. Not we have a function which would skip
serialization/deserializaton for the unit if we forgot to set either of the
fields.

src/core/unit.c

@@ -3488,12 +3488,6 @@ void unit_unwatch_bus_name(Unit *u, const char *name) {
         u->get_name_owner_slot = sd_bus_slot_unref(u->get_name_owner_slot);
 }
 
-bool unit_can_serialize(Unit *u) {
-        assert(u);
-
-        return UNIT_VTABLE(u)->serialize && UNIT_VTABLE(u)->deserialize_item;
-}
-
 int unit_add_node_dependency(Unit *u, const char *what, UnitDependency dep, UnitDependencyMask mask) {
         _cleanup_free_ char *e = NULL;
         Unit *device;