#1055 Remove multiple "please register" messages

Part 1:
- Use only one message entry for "Please register", that may have to be adapted to reflect the proper /register arguments
- Remove other message entries from code and from the messages files

Breaking change: reg_email_msg is also removed

src/main/java/fr/xephi/authme/command/executable/email/RecoverEmailCommand.java

@@ -59,7 +59,7 @@ public class RecoverEmailCommand extends PlayerCommand {
 
         PlayerAuth auth = dataSource.getAuth(playerName); // TODO: Create method to get email only
         if (auth == null) {
-            commonService.send(player, MessageKey.REGISTER_EMAIL_MESSAGE);
+            commonService.send(player, MessageKey.USAGE_REGISTER);
             return;
         }