Fix "Make it compile against last qt5.15 without deprecated method. QProcess::execute(QString) is deprecated"

Summary:
This port is broken AFAICT - it tries to run the full Exec= line as binary, without
splitting arguments first.

This reverts commit 59cbea835502428f30c1495abe4a1b3d133103e3.

Test Plan: Builds fine.

Reviewers: Frameworks, mlaurent

Reviewed By: mlaurent

Subscribers: ahmadsamir, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D27863