IfcRelAssigns WR1 and attribute 6 is obsolete and should be removed

Description of the proposal:

See [IfcRelAssigns](https://standards.buildingsmart.org/IFC/DEV/IFC4_2/FINAL/HTML/link/ifcrelassigns.htm).

WR1 states:

```
WR1 : IfcCorrectObjectAssignment(RelatedObjectsType, RelatedObjects);
```

Attribute 6 documentation states:

> IFC4 CHANGE The attribute is deprecated and shall no longer be used. A NIL value should always be assigned.

While we're at it, I propose to remove _both_ WR1 and attribute 6, since it's always NULL anyway.

Describe how it contributes to the objectives (https://github.com/buildingSMART/NextGen-IFC/wiki/Towards-a-technology-independent-IFC):

Is this a proposal to 'add', 'remove' of 'change' entities in the schema (pick one):

Change

What do we win:

Consistent and correct docs

What do we loose

Nothing

Schema impact:

WR1 removal. Attribute removal.
Instance model impact: ?

Backwards compatible: No

Automatic migration possible: Yes

Additional implications:

-
Note that not all points need to be satisfied!
Backwards compatibility and file size are not concerns.