Only build activities support if QtSQL is found

Summary: Check if QtSql is found before including the activities directory. Drop unneeded find_package calls

Test Plan: builds

Reviewers: #frameworks, apol

Reviewed By: apol

Subscribers: apol, kde-frameworks-devel, kfm-devel

Tags: #dolphin, #frameworks

Differential Revision: https://phabricator.kde.org/D25528