xwayland: Make launcher not force EGL_PLATFORM

It was added to work around Xwayland crashing at startup due to a mesa
issue, see commit 58fa92e40b.

It's been almost 6 years since that change. No other major wayland
compositor forces EGL_PLATFORM, so lets remove the workaround.

The main motivation behind this change is to get rid of code that's not
straightforward and should be unnecessary in general.

src/xwl/xwaylandlauncher.cpp

@@ -167,7 +167,6 @@ bool XwaylandLauncher::startInternal()
     m_xwaylandProcess->setProgram(QStringLiteral("Xwayland"));
     QProcessEnvironment env = QProcessEnvironment::systemEnvironment();
     env.insert("WAYLAND_SOCKET", QByteArray::number(wlfd));
-    env.insert("EGL_PLATFORM", QByteArrayLiteral("DRM"));
     if (qEnvironmentVariableIsSet("KWIN_XWAYLAND_DEBUG")) {
         env.insert("WAYLAND_DEBUG", QByteArrayLiteral("1"));
     }