[libSyntax] Remove incremental JSON transfer option

We were only keeping track of `RawSyntax` node IDs to incrementally transfer a syntax tree via JSON. However, AFAICT the incremental JSON transfer option has been superceeded by `SyntaxParseActions`, which are more efficient.

So, let’s clean up and remove the `RawSyntax` node ID and JSON incremental transfer option.

In places that still need a notion of `RawSyntax` identity (like determining the reused syntax regions), use the `RawSyntax`’s pointer instead of the manually created ID.

In `incr_transfer_round_trip.py` always use the code path that uses the `SyntaxParseActions` and remove the transitional code that was still using the incremental JSON transfer but was never called.

utils/gyb_sourcekit_support/UIDs.py

@@ -454,7 +454,6 @@ UID_KINDS = [
     KIND('StatNumRequests', 'source.statistic.num-requests'),
     KIND('StatNumSemaRequests', 'source.statistic.num-semantic-requests'),
     KIND('SyntaxTreeOff', 'source.syntaxtree.transfer.off'),
-    KIND('SyntaxTreeIncremental', 'source.syntaxtree.transfer.incremental'),
     KIND('SyntaxTreeFull', 'source.syntaxtree.transfer.full'),
     KIND('Swift', 'source.lang.swift'),
     KIND('ObjC', 'source.lang.objc'),