[Distributed] Remove _distributedThunkTarget; it is not necessary (#72245)

lib/Serialization/Serialization.cpp

@@ -3065,9 +3065,6 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
           S.addDeclRef(afd), pieces.size(), pieces);
       return;
     }
-    case DeclAttrKind::DistributedThunkTarget: {
-      assert(false && "not implemented");
-    }
 
     case DeclAttrKind::TypeEraser: {
       auto abbrCode = S.DeclTypeAbbrCodes[TypeEraserDeclAttrLayout::Code];