Revert "Install translated documentation from po/ too"

Temporarily revert it, as it leads to an error:
  add_custom_target cannot create target
  "po-fr-docs-plasma-sdk-plasmoidviewer-plasmoidviewer-1" because another
  target with the same name already exists.  The existing target is a custom
  target created in source directory

which should have been visible even before, need to investigate.

This reverts commit 7ca5760e.

(cherry picked from commit 4ef63918)