Remove redundant and unused message

- Remove 'reg_voluntarily'
- Remove 'user_unknown' in favor of 'unknown_user' - but move text from old 'user_unknown' to 'unknown_user'

Found by @Twonox

src/main/java/fr/xephi/authme/command/executable/authme/AccountsCommand.java

@@ -56,7 +56,7 @@ public class AccountsCommand implements ExecutableCommand {
 
                     List<String> accountList = dataSource.getAllAuthsByIp(auth.getIp());
                     if (accountList.isEmpty()) {
-                        commonService.send(sender, MessageKey.USER_NOT_REGISTERED);
+                        commonService.send(sender, MessageKey.UNKNOWN_USER);
                     } else if (accountList.size() == 1) {
                         sender.sendMessage("[AuthMe] " + playerName + " is a single account player");
                     } else {

src/main/java/fr/xephi/authme/command/executable/authme/LastLoginCommand.java

@@ -29,7 +29,7 @@ public class LastLoginCommand implements ExecutableCommand {
 
         PlayerAuth auth = dataSource.getAuth(playerName);
         if (auth == null) {
-            commonService.send(sender, MessageKey.USER_NOT_REGISTERED);
+            commonService.send(sender, MessageKey.UNKNOWN_USER);
             return;
         }