Remove "Object Extractors"

[rbuckton]

Based on numerous conversations with the Pattern Matching proposal champions, we have decided to drop support for "Object Extractors" (i.e., Point{ x, y } = p) from the proposal. The reason being that Object Extractors carve off far too much usable syntax space by taking up all of Identifier `{`, and that object extraction can still be achieved using "Array Extractors" via Point({ x, y }) = p.

Per these changes, Object Extractors should be removed from the explainer going forward.