#539 Remove preference mail_sender_into_to

[Markus Fassbender]

Since we set "undisclosed recipients" in the from field with version 2.4.5 there is no need to add a user email to the to field if we send a mail to a role.

We should watch the response of version 2.4.5 where we disable this preference per default and if no problems occure then we remove this preference.

[Markus Fassbender]

We should remove this later because Some providers don't allow "undisclosed recipients" äs á valid email address.