remove unused class SwXMLIgnoreSectionListContext

Change-Id: Ia71942a7789a2c4358a4cc2b9744cddaa4544eef
Reviewed-on: https://gerrit.libreoffice.org/13289
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>