XLSX: don't use sheetId attribute from workbook.xml to link a sheet name to its filename. The first sheet is sheet1.xml, etc...

git-svn-id: https://svn.osgeo.org/gdal/trunk@26895 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx