[Syntax] Remove LegacyASTTransformer

As it is no longer ever instantiated, and since Syntax nodes are being
plumbed through the parser, the LegacyASTTransformer no longer needs to
exist.

Author: harlanhaskins

include/swift/AST/LegacyASTTransformer.h

@@ -24,10 +24,6 @@
 
 namespace swift {
 
-namespace syntax {
-  class LegacyASTTransformer;
-}
-
 /// The top-level container and manager for semantic analysis.
 ///
 /// Eventually, this should contain cached semantic information such as

include/swift/AST/SyntaxASTMap.h

@@ -54,7 +54,6 @@ const auto NoParent = llvm::None;
 /// their children.
 class Syntax {
   friend struct SyntaxFactory;
-  friend class LegacyASTTransformer;
   friend class swift::SyntaxASTMap;
 
 protected:

include/swift/Syntax/Syntax.h

@@ -33,7 +33,6 @@ add_swift_library(swiftAST STATIC
   GenericSignatureBuilder.cpp
   Identifier.cpp
   LayoutConstraint.cpp
-  LegacyASTTransformer.cpp
   LookupVisibleDecls.cpp
   Module.cpp
   ModuleLoader.cpp