Remove dysfunctional convenience methods from Item/Property

[thiemowmde]

this._statementGroupSet is, as the name suggests, a StatementGroupSet, which is a Set. The Set class declares both an isEmpty and an equals method, but there is no addStatement or removeStatement anywhere in the code base. Not declared, not tested, and not used anywhere.

To add and remove statements code calls entity.getStatements().addItem( … ) and removeItem( … ), both declared on the Set class.