No longer export QT_QPA_PLATFORM env variable to the session

Summary:
As discussed the env variables are no longer exported. Thus Qt
applications follow the default qpa platform they are compiled with and
thus still function if they are packaged with a Qt without QtWayland.
Plasma's internal processes pick the qpa platform depending on the
session type as well as our flatpak apps.

KRunner and Plasmashell are adjusted to not leak the env variable they
set for themselves.

Test Plan:
Started a wayland session, verified with KWin's debug console
that plasmashell and krunner are wayland. Launched kwrite from both plasma
and krunner and verified that it's xcb

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D11447