NFC: Remove outdated comment

Author: hamishknight

include/swift/Subsystems.h

@@ -60,12 +60,6 @@ namespace swift {
   class LangOptions;
   class SILOptions;
   class ModuleDecl;
-  /// A opaque syntax node created by a \c SyntaxParseAction, whose contents
-  /// must be interpreted by the \c SyntaxParseAction which created it.
-  /// Requires the two low bits to be 0, so that it can be stored in an
-  /// \c llvm::PointerIntPair. This is in particular guaranteed for pointers
-  /// to C/C++ objects and for pointers that were generated by Swift and passed
-  /// to the compiler via a C API (in particular \c CLibParseActions ).
   class Parser;
   class SerializationOptions;
   class SILOptions;