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/resources/messages/messages_en.yml

@@ -2,10 +2,9 @@ denied_command: '&cIn order to use this command you must be authenticated!'
 same_ip_online: 'A player with the same IP is already in game!'
 denied_chat: '&cIn order to chat you must be authenticated!'
 kick_antibot: 'AntiBot protection mode is enabled! You have to wait some minutes before joining the server.'
-unknown_user: '&cCan''t find the requested user in the database!'
+unknown_user: '&cThis user isn''t registered!'
 unsafe_spawn: '&cYour quit location was unsafe, you have been teleported to the world''s spawnpoint.'
 not_logged_in: '&cYou''re not logged in!'
-reg_voluntarily: 'You can register yourself to the server with the command "/register <password> <ConfirmPassword>"'
 usage_log: '&cUsage: /login <password>'
 wrong_pwd: '&cWrong password!'
 unregistered: '&cSuccessfully unregistered!'
@@ -22,7 +21,6 @@ login_msg: '&cPlease, login with the command "/login <password>"'
 reg_msg: '&3Please, register to the server with the command "/register <password> <ConfirmPassword>"'
 usage_unreg: '&cUsage: /unregister <password>'
 pwd_changed: '&2Password changed successfully!'
-user_unknown: '&cThis user isn''t registered!'
 password_error: '&cPasswords didn''t match, check them again!'
 password_error_nick: '&cYou can''t use your name as password, please choose another one...'
 password_error_unsafe: '&cThe chosen password isn''t safe, please choose another one...'