*: 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

24565:27:27

running ERP5.UnitTest-Master.Medusa

24565:27:25

running SlapOS.Eggs.UnitTest-Master.Python2

24565:27:40

running SlapOS.Eggs.UnitTest-Master.Python3

24565:27:48