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/message/MessageKey.java

@@ -26,9 +26,6 @@ public enum MessageKey {
     /** You're not logged in! */
     NOT_LOGGED_IN("not_logged_in"),
 
-    /** You can register yourself to the server with the command "/register <password> <ConfirmPassword>" */
-    REGISTER_VOLUNTARILY("reg_voluntarily"),
-
     /** Usage: /login <password> */
     USAGE_LOGIN("usage_log"),
 
@@ -77,9 +74,6 @@ public enum MessageKey {
     /** Password changed successfully! */
     PASSWORD_CHANGED_SUCCESS("pwd_changed"),
 
-    /** This user isn't registered! */
-    USER_NOT_REGISTERED("user_unknown"),
-
     /** Passwords didn't match, check them again! */
     PASSWORD_MATCH_ERROR("password_error"),