Remove export for all the netpbm formats.
Import still works but these filetypes don't preserve good quality
anyway.

Change-Id: If4b47f749275a9d3d663f61872f448e01112f6b5
Reviewed-on: https://gerrit.libreoffice.org/20361Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>