Removed some useless workaround for type definitions.

Fixes https://github.com/mono/CppSharp/issues/671.

src/AST/ASTVisitor.cs

@@ -390,9 +390,6 @@ namespace CppSharp.AST
             if (!VisitDeclaration(typedef))
                 return false;
 
-            if (typedef.Type == null)
-                return false;
-
             return typedef.Type.Visit(this, typedef.QualifiedType.Qualifiers);
         }
 
@@ -401,9 +398,6 @@ namespace CppSharp.AST
             if (!VisitDeclaration(typeAlias))
                 return false;
 
-            if (typeAlias.Type == null)
-                return false;
-
             return typeAlias.Type.Visit(this, typeAlias.QualifiedType.Qualifiers);
         }