core/serialization: drop misleadingly-named unit_can_serialize()

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.