[startplasma] don't set QT_AUTO_SCREEN_SCALE_FACTOR with Qt >= 5.14

Summary:
It gives the following runtime warning:

Warning: QT_AUTO_SCREEN_SCALE_FACTOR is deprecated. Instead use:
   QT_ENABLE_HIGHDPI_SCALING to enable platform plugin controlled per-screen factors

But we don't need to set QT_ENABLE_HIGHDPI_SCALING to 0.
Just setting QT_SCREEN_SCALE_FACTORS is enough to control scaling, after
my commit e018d11600bffc6 to qtbase.

Kudos for porting startkde to be C++ code, it makes this commit much
easier...

Test Plan: Builds; not rebooted yet

Reviewers: apol, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24255