*: remove remaining bits of old PDF preview

The action was removed in 6dce55b0 (dms: drop PDF thumbnail view, 2021-06-01)
but some CSS declarations were not removed.

Also remove some old commented out javascript that was referencing the
old PDF preview
Status Job ID Name Coverage
  External
running ERP5.UnitTest-Master

22653:06:56

running ERP5.UnitTest-Master.Medusa

22653:06:54

running SlapOS.Eggs.UnitTest-Master.Python2

22653:07:08

running SlapOS.Eggs.UnitTest-Master.Python3

22653:07:17