shell: don't set show close button on infobar
This is a leftover from d269d4ad. Settting the close button is the default since that commit
Avoids:
Gtk-CRITICAL **: 19:38:23.643: gtk_info_bar_set_show_close_button: assertion 'GTK_IS_INFO_BAR (info_bar)' failed
It is possible to reproduce whenevern trying to print and then cancelling the operation.