gnunnet: don't copy non-existing files

gnunnet: don't copy non-existing files

There are six places pointing to files that do not exist any more:
 - gns-import.sh in package gnunet-gns (dropped in v0.11.0)
 - libgnunetdnsstub.so* in gnunet-vpn (integrated into util in v0.11.0)
 - libgnunettun.so* in gnunet-vpn (integrated into util in v0.11.0)
 - gnunet-service-ats-new in package gnunet (dropped in v0.12.0)
 - libgnunetreclaimattribute.so.* (integrated into reclaim in v0.13.0)
 - libgnunetabe.so.* in gnunet-reclaim (dropped in v0.17.2)

They were not noticed because their failing copy commands were part of
loops in which only the last operation had its exit status checked.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>