Clean up some code quality issues

This removes some qualifiers that aren't necessary (e.g. public/abstract on interfaces, private on enum constructors, final on private methods, static on nested interfaces/enums), uses Java 8 lambdas and features where that's an improvement

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177182945

java/google/registry/model/common/EntityGroupRoot.java

@@ -40,7 +40,7 @@ public class EntityGroupRoot extends BackupGroupRoot {
   private String id;
 
   /** The root key for cross-tld resources such as registrars. */
-  public static final Key<EntityGroupRoot> getCrossTldKey() {
+  public static Key<EntityGroupRoot> getCrossTldKey() {
     return Key.create(EntityGroupRoot.class, "cross-tld");
   }
 }