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.
9 jobs for fix/drop_PDF_thumbnails_view in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-TestRunner1

05:52:58

passed ERP5.PerformanceTest-TestRunner1

00:30:04

failed ERP5.UnitTest-TestRunner1

04:45:37

passed ERP5.CodingStyleTest-TestRunner1

00:49:02

passed ERP5.PerformanceTest-TestRunner1

00:24:22

failed ERP5.UnitTest-TestRunner1

01:51:02

canceled ERP5.UnitTest-TestRunner1

failed ERP5.UnitTest-TestRunner1

01:20:08

failed ERP5.UnitTest-TestRunner1