swift234

SpriteKit overlay: remove the subscript from SKNode

In iOS 8.0/OS X 10.10, SpriteKit introduced an
-objectForKeyedSubscript: method that enabled subscripting in
Objective-C. However, this subscript wasn't generally useful in Swift
because it took AnyObject and produced AnyObject, so the SpriteKit
overlay defined a subscript that took a String and produced an
[SKNode].

Subsequently, in iOS 9.0/OS X 10.10, -objectForKeyedSubscript: got
type annotations that gave it the appropriate Swift signature, which
made the subscript defined in the overlay redundant.

The twisty logic of the Clang importer would suppress the imported
subscript when it saw the one in the overlay, hiding the
redundancy. My cleanup of that logic in
0c0a0fab4b8c2a824882fd1dd68379f94a878055 caused uses of the subscript
to be redundant.

Removal of the redundant code in the overlay is the overall best
answer, because it minimizes the size of the overlay and leaves the
API in the Objective-C header. However, this will introduce a
regression for SpriteKit applications targeting iOS 7.0/OS X 10.8,
where the overlay was compensating for the lack of this operation
before iOS 8.0 / OS X 10.8. There are workarounds here we can
investigate, although they're fairly hacky.