Drop unnecessary dependency on KF5DocTools

Summary:
No need for it anymore after b2198b447ab2c4d048035aeacd71a8b05f3c53df
removed the SMTP kioslave in favour of the the ksmtp library.

Test Plan: Builds fine.

Reviewers: KDE PIM, dvratil

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D8839