dms: drop PDF thumbnail view

Since 7f32f8cd (erp5_dms: Add PDF Reader using the pdf.js, 2016-06-24)
we have a PDF preview with a javascript PDF view, which is much better way
of viewing PDF.

This commit made the Thumbnail preview obsolete, also it does not really
work on ERP5JS, so remove the thumbnail preview.
3 jobs for fix/drop_PDF_thumbnails_view in 0 seconds
Status Job ID Name Coverage
  External
canceled ERP5.CodingStyleTest-TestRunner1

00:40:15

canceled ERP5.PerformanceTest-TestRunner1

00:38:56

canceled ERP5.UnitTest-TestRunner1

00:40:13