[index] Remove unreachable materializeForSet case in indexer

We already had it marked unreachable in SourceKit, so lift that up into
the libIndex and remove the unnecessary enum case.

include/swift/Index/IndexSymbol.h

@@ -64,7 +64,6 @@ enum class SymbolSubKind {
   AccessorSetter,
   AccessorWillSet,
   AccessorDidSet,
-  AccessorMaterializeForSet,
   AccessorAddressor,
   AccessorMutableAddressor,