[turbofan] VisitSuperCallReference is not reachable

BUG=N
LOG=N
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1226443004

Cr-Commit-Position: refs/heads/master@{#29522}

src/compiler/ast-graph-builder.cc

@@ -2844,8 +2844,8 @@ void AstGraphBuilder::VisitSuperPropertyReference(
 
 
 void AstGraphBuilder::VisitSuperCallReference(SuperCallReference* expr) {
-  Node* value = BuildThrowUnsupportedSuperError(expr->id());
-  ast_context()->ProduceValue(value);
+  // Handled by VisitCall
+  UNREACHABLE();
 }