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.
1 job for fix/component-editor
Status Job ID Name Coverage
  External
failed ERP5.UnitTest-Master