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/tools/EppToolCommand.java

@@ -133,7 +133,7 @@ abstract class EppToolCommand extends ConfirmingCommand implements ServerSideCom
     String prompt = addHeader("Command(s)", Joiner.on("\n").join(commands)
         + (force ? "" : addHeader("Dry Run", Joiner.on("\n").join(processCommands(true)))));
     force = force || isDryRun();
-    return prompt.toString();
+    return prompt;
   }
 
   private List<String> processCommands(boolean dryRun) throws IOException {