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/rde/imports/RdeHostLinkAction.java

@@ -204,7 +204,7 @@ public class RdeHostLinkAction implements Runnable {
     }
   }
 
-  private static enum HostLinkResult {
+  private enum HostLinkResult {
     HOST_NOT_FOUND,
     HOST_OUT_OF_ZONE,
     SUPERORDINATE_DOMAIN_IN_PENDING_DELETE,