app: stop setting QT_QPA_PLATFORM=wayland

app: stop setting QT_QPA_PLATFORM=wayland

Some environments, eg. Ubuntu 22.04, have issues with copy/paste when
QT_QPA_PLATFORM=wayland is configured.

Simplify things by removing our management of this variable.
Document its existence and let things use default values so that things
are better behaved in more scenarios.

This part of the code was originally authored to eliminate a warning
that Qt would emit on older versions of debian/testing:

    Warning: Ignoring XDG_SESSION_TYPE=wayland on Gnome.
    Use QT_QPA_PLATFORM=wayland to run on Wayland anyway.

That warning was related to https://bugreports.qt.io/browse/QTBUG-68619
and is no longer relevant.

It's better to let Qt use its defaults since users and distribution
may have customizations that are harmed by our attempt to enable the
wayland QPA platform plugin.

Closes: #1295
Reported-by: Jakub Klos <jakub@icewarp.com>
Signed-off-by: David Aguilar <davvid@gmail.com>