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.