Revert "Fixed bug #6, explicitly set the QT platform for the shell"

Now that we have Qt's platform plugin for Wayland, we can use that
for the shell. This is a step towards not needing X11 at all.

Test with:

    Xephyr :2 &
    DISPLAY=:2 flatpak run com.valvesoftware.SteamLink//master

Previously, both the shell (UI layer) and the actual streaming client
would run on the Xephyr display. After this change, the shell runs
full-screen again (via Wayland), and the actual streaming client uses
X11 by default.

The streaming client can be forced to use Wayland too by setting
environment variable SDL_VIDEODRIVER=wayland.

This reverts commit 9845f4f.

Signed-off-by: Simon McVittie <smcv@collabora.com>

com.valvesoftware.SteamLink.yml

@@ -8,7 +8,6 @@ finish_args:
   - --device=all
   - --env=LD_LIBRARY_PATH=/app/lib
   - --env=SDL_GAMECONTROLLERCONFIG_FILE=/var/data/Valve Corporation/SteamLink/controller_map.txt
-  - --env=QT_QPA_PLATFORM=xcb
   - --share=ipc
   - --share=network
   - --socket=pulseaudio