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/test/java/fr/xephi/authme/process/login/AsynchronousLoginTest.java

@@ -98,7 +98,7 @@ public class AsynchronousLoginTest {
 
         // then
         verify(playerCache, only()).isAuthenticated(name);
-        verify(commonService).send(player, MessageKey.USER_NOT_REGISTERED);
+        verify(commonService).send(player, MessageKey.UNKNOWN_USER);
         verify(dataSource, only()).getAuth(name);
     }