Remove ModelStatement.statementSource()

ModelStatement.statementSource() is a confusing contract, as it allows for DeclaredStatements which are not StatementOrigin.DECLARATION. A DeclaredStatement is by definition declared, so this does not quite make sense.

Based on historical record, the interface design comes from 2015 (Java 7). In Java 8 world, though, and thus have a consistent way of saying that:

• DeclaredStatement is always StatementOrigin.DECLARATION
• EffectiveStatement is either one of the possibilities based on getDeclared() being null (or not)

Remove all ModelStatement.statementSource() implementations in favor of default implementations in DeclaredStatement and EffectiveStatement.