backends/x11: Remove AbstractEglBackend::setSurface() call

KWin requires surfacesless contexts, so this setSurface() is not needed.
This ensures that makeCurrent() won't make the opengl context current
against a surface that belongs to a removed output.

src/backends/x11/windowed/egl_x11_backend.cpp

@@ -103,7 +103,6 @@ bool X11WindowedEglBackend::createSurfaces()
     if (m_outputs.isEmpty()) {
         return false;
     }
-    setSurface(m_outputs.first()->surface());
     return true;
 }