session: reset session type to default when selecting non-wayland session
    
    If we select a non-wayland session we need to clear the session type,
    or it will linger as a wayland session if it was one previously.
    
    Prevents users from getting logged into an X session when with
    XDG_SESSION_TYPE=wayland.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=745235