Do not rely on documentInfo.pageCount when selecting which pages to

scrap

documentInfo.PageCount might be set to "unknown" == -1. Hence we don't
know where the last page to scrap ends. Hence defer this decision all
way until PdfManipulationService.removePages as then we know the actual
pages of the document.

Change-Id: I063c3cd084d65a3ac5c60c3d85cec3a346be2680
Fixed: 27948632