#1055 Remove multiple "please register" messages

Part 1:
- Use only one message entry for "Please register", that may have to be adapted to reflect the proper /register arguments
- Remove other message entries from code and from the messages files

Breaking change: reg_email_msg is also removed

src/test/java/fr/xephi/authme/command/executable/email/RecoverEmailCommandTest.java

@@ -118,7 +118,7 @@ public class RecoverEmailCommandTest {
         verify(sendMailSsl).hasAllInformation();
         verify(dataSource).getAuth(name);
         verifyNoMoreInteractions(dataSource);
-        verify(commandService).send(sender, MessageKey.REGISTER_EMAIL_MESSAGE);
+        verify(commandService).send(sender, MessageKey.USAGE_REGISTER);
     }
 
     @Test