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This reverts commit e15cd5ec.

One of the plugins links against KIOWidgets, which means that CMake will
fail configuration if KIO is not found.

Instead of using a KIO Slave for sending emails via SMTP, for which
KIO wasn't really designed, use a KSMTP library. This has no
functional or performance impacts, but improves maintainability
of our SMTP code and opens doors to further improvements (like
native Gmail authentication)

Differential Revision: https://phabricator.kde.org/D6879