Do not force QT_WAYLAND_DISABLE_WINDOWDECORATION=1 on wayland greeters

This would put Qt in a weird state (see the issue that we are fixing)
where it would use the wrong GL configuration.

Arguably it's an error of Qt for having a bogus default pixel format but
this change should fix it but also make sure it behaves like other Qt
applications and thus more predictably.

Fixes #1441

src/daemon/Greeter.cpp

@@ -207,7 +207,6 @@ namespace SDDM {
                 m_auth->setCookie(qobject_cast<XorgDisplayServer*>(displayServer)->cookie());
             } else if (m_display->displayServerType() == Display::WaylandDisplayServerType) {
                 env.insert(QStringLiteral("QT_QPA_PLATFORM"), QStringLiteral("wayland"));
-                env.insert(QStringLiteral("QT_WAYLAND_DISABLE_WINDOWDECORATION"), QStringLiteral("1"));
                 env.insert(QStringLiteral("QT_WAYLAND_SHELL_INTEGRATION"), QStringLiteral("fullscreen-shell-v1"));
             }
             m_auth->insertEnvironment(env);