[TASK] removing SMTP documentation

removing the SMTP config in the documentation,
since mail sending is handled by swiftmailer.

Resolves: #58160
Releases: 4.0
Change-Id: I38b5bf55958209acaa7e865eaddedfd3f02c8db9
Reviewed-on: https://review.typo3.org/31279
Reviewed-by: Ivan Dharma Kartolo <ivan.kartolo@gmail.com>
Tested-by: Ivan Dharma Kartolo <ivan.kartolo@gmail.com>