Refactor Guava functional methods to use lambdas

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177027488

java/google/registry/util/DatastoreServiceUtils.java

@@ -15,15 +15,11 @@
 package google.registry.util;
 
 import com.google.appengine.api.datastore.Key;
-import com.google.common.base.Function;
 import java.util.Optional;
 
 /** Utility methods for working with the App Engine Datastore service. */
 public class DatastoreServiceUtils {
 
-  /** Helper function that extracts the kind from a regular Datastore entity key. */
-  public static final Function<Key, String> KEY_TO_KIND_FUNCTION = Key::getKind;
-
   /** Returns the name or id of a key, which may be a string or a long. */
   public static Object getNameOrId(Key key) {
     return Optional.<Object>ofNullable(key.getName()).orElse(key.getId());