Remove the Reference property in the common property sets

Description of the proposal:

Not exactly affecting the schema, but does affect Pset definition templates, and definitely affects the users experience as it removes an important ambiguity that I've seen come up again and again. Basically the defined by type `Name` attribute and the `Pset_*Common.Reference` is a duplicate, seemingly as a fallback for software that doesn't implement IFC types. I propose to remove the duplication and stick to `Name`, which is the natural identification field people look for and has always been recommended in the past.

More info: https://forums.buildingsmart.org/t/how-do-you-store-element-codes-in-ifc/884
More more info: https://standards.buildingsmart.org/documents/Implementation/IFC_Implementation_Agreements/CV-2x3-136.html

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

Less complexity, as there is a clear rule of where data should be stored

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

Remove

What do we win:

Less complexity

What do we lose

Compatibility with software that doesn't support type relationships

Schema impact:

Pset definition template removed

Instance model impact: ?

Backwards compatible:

Yes

Automatic migration possible:

Yes

Additional implications:

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