Do not set short absence message for system autoresponder

[ChristophWurst]

Steps to reproduce

1. Configure an absence
2. Go absent
3. Receive an email

Expected behavior

Sender receives an automated message with "Re: xxx".

Actual behavior

Sender receives an automated message with my user status.

Mail app version

3.5

Mailserver or service

No response

Operating system

No response

PHP engine version

None

Web server

None

Database

None

Additional info

Discussed in #8925 (comment)

[ChristophWurst]

Once fixed let's also reflect this in the user docs nextcloud/documentation#11408.

[st3iny]

The default subject is defined as follows:

   Users can specify the Subject of the reply with the ":subject"
   parameter.  If the :subject parameter is not supplied, then the
   subject is generated as follows: The subject is set to the characters
   "Auto: " followed by the original subject.  An appropriate fixed
   Subject, such as "Automated reply", SHOULD be used in the event that
   :subject isn't specified and the original message doesn't contain a
   Subject field.

E.g. Auto: Subject blah blah

Ref https://www.rfc-editor.org/rfc/rfc5230.html#section-5.3

[ChristophWurst]

Then let's use this default subject 👍