Remove unnecessary override in ReplacementVisitor.

R=brianwilkerson@google.com

Change-Id: Iba88984097212242e1373e80af61a340e22d4a1f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153500
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: scheglov

pkg/analyzer/lib/src/dart/element/replacement_visitor.dart

@@ -102,11 +102,6 @@ class ReplacementVisitor
     );
   }
 
-  @override
-  DartType visitDartType(DartType type) {
-    return null;
-  }
-
   @override
   DartType visitDynamicType(DynamicType type) {
     return null;