Code review comment for lp:~vanvugt/mir/remove-unused-session-param

Yes, unity-mir will need fixing, but it doesn't appear to have a functional use for "session":

$ grep -r requestPlacementFor *
src/unity-mir/initialsurfaceplacementstrategy.cpp: Q_EMIT requestPlacementForSession(&session, x, y);
src/unity-mir/initialsurfaceplacementstrategy.h: void requestPlacementForSession(mir::shell::Session const* session, uint32_t &x, uint32_t &y);
src/modules/Unity/Application/application_manager.cpp: QObject::connect(m_mirServer->placementStrategy(), &InitialSurfacePlacementStrategy::requestPlacementForSession,

Unless it's in a different project?