Remove duplicate QT_REQUIRED_VERSION from autotests

Summary:

1. It's bound to differ from the QT_REQUIRED_VERSION in the root CMakeLists.txt
2. It seems to be matched by the script increasing the versions and going from 5.6.0 to 5.6.1 and possibly 5.6.2 and .3 in the future doesn't seem intentional at all.

Test Plan: cmake output shows: "Qt5Test (required version >= 5.7.0)"

Reviewers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D7703