Remove the login button for guest users commenting

After discussing it with Vincent, it seems like this button is not really necessary. It has two main uses:

1. Give a hint to the user why they need to fill up a name for their comment : “You’re logged out”
2. Allow to quickly login before posting a comment.

`2.` is wrong because it assumes the user can login (or register). From our discussion, the reason why guests would be able to comment on pages would often be that the regular wiki “users” are not actually supposed to log in. In this case, we make a promise that cannot be fulfilled. By clicking the button the user expects to be able to log in, but there’s no way for them to get their hands on an account.

`1.` could be done with a regular text hint.