Remove the type parameter from NullabilityEliminator.perform().

R=brianwilkerson@google.com

Change-Id: Iebf32f926aec6d904940d531a8c56318f33b8074
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153483
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: scheglov

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

@@ -37,7 +37,7 @@ class NullabilityEliminator extends ReplacementVisitor {
   /// If the [type] itself, or any of its components, has any nullability,
   /// return a new type with legacy nullability suffixes. Otherwise return the
   /// original instance.
-  static T perform<T extends DartType>(TypeProviderImpl typeProvider, T type) {
+  static DartType perform(TypeProviderImpl typeProvider, DartType type) {
     if (type == null) {
       return type;
     }