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/command/executable/authme/AccountsCommandTest.java

@@ -89,7 +89,7 @@ public class AccountsCommandTest {
         runInnerRunnable(bukkitService);
 
         // then
-        verify(service).send(sender, MessageKey.USER_NOT_REGISTERED);
+        verify(service).send(sender, MessageKey.UNKNOWN_USER);
         verify(sender, never()).sendMessage(anyString());
     }
 

src/test/java/fr/xephi/authme/command/executable/authme/LastLoginCommandTest.java

@@ -54,7 +54,7 @@ public class LastLoginCommandTest {
 
         // then
         verify(dataSource).getAuth(player);
-        verify(service).send(sender, MessageKey.USER_NOT_REGISTERED);
+        verify(service).send(sender, MessageKey.UNKNOWN_USER);
     }
 
     @Test

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);
     }