hyperschema: remove obsolete references to core terminology definitions

[dlax]

At the beginning of hyperschema document, there's:

The terms "schema", "instance", "property" and "item" are to be interpreted as defined in the JSON Schema core specification.

but core specification does no longer defines "property" and "item" (from 3cbfc8b AFAICT). I guess the former is assumed by JSON terminology; the latter may be confusing w.r.t. to collection+item terminology that appears in sections about links. So removing both references.

[handrews]

I checked the validation schema and it doesn't try to reference or define these either, although it uses the terms in the conventional manner.

So I agree that we don't need to define them here, and should definitely remove the dangling references.

[handrews]

@dlax feel free to update this to add yourself to the acknowledgement section

[dlax]

@dlax feel free to update this to add yourself to the acknowledgement section

@handrews done in 627801e.

[Relequestual]

OK by me!