Remove cardinality checks for attributes

It is possible to specify cardinality for attributes as shown in [this](https://github.com/buildingSMART/IDS/blob/master/Documentation/testcases/ids/pass-specification_optionality_and_facet_optionality_can_be_combined.ids) example.

The specific requirement I'm talking about is this:
```


Description


Foobar


```

The interpretation of this is vague. I'd remove the possibility to specify cardinality for attributes. If IDS says something about an attribute, it is a requirement, not optional.