Get rid of Providers class

inject_common isn't going to be open-sourced anytime soon, so replace it with a
lambda, which wasn't an option when this was originally written we were still
on Java 7 back then.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=213318557

javatests/google/registry/testing/Providers.java

@@ -1,29 +0,0 @@
-// Copyright 2017 The Nomulus Authors. All Rights Reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package google.registry.testing;
-
-import javax.inject.Provider;
-
-/** Helper functions for {@link Provider}. */
-public final class Providers {
-
-  /**
-   * Returns a {@link Provider} that supplies a constant value.
-   *
-   */
-  public static <T> Provider<T> of(final T instance) {
-    return () -> instance;
-  }
-}