[ASTGen] Remove LegacyParser ping-pong mechanism

Now that ASTGen should be able to generate most Swift code. Let's
remove "legacy parser" call-in, and remove the unhealthy cyclic
dependency between lib/Parse and ASTGen.

include/module.modulemap

@@ -16,12 +16,6 @@ module IDEBridging {
   export *
 }
 
-module ParseBridging {
-  header "swift/Parse/ParseBridging.h"
-  requires cplusplus
-  export *
-}
-
 module SILBridging {
   header "swift/SIL/SILBridging.h"
   requires cplusplus