GRMLBASE/98-clean-chroot: get rid of /wget-log* files

The update-pciids run under timeout(1) within
/etc/grml/fai/config/scripts/GRMLBASE/91-update-pciids
might fail and leave /wget-log* files behind, like:

| root@gf-internal ~ # cat /srv/grml-live/grml64-forensic_2020.01-1/grml_chroot/wget-log
| http://pciids.sourceforge.net/v2.2/pci.ids.bz2:
| 2019-12-10 15:02:31 ERROR 503: Service Temporarily Unavailable.
| root@gf-internal ~ # cat /srv/grml-live/grml64-forensic_2020.01-1/grml_chroot/wget-log.1
| http://pciids.sourceforge.net/v2.2/pci.ids.bz2:
| 2019-12-10 15:07:44 ERROR 503: Service Temporarily Unavailable.

This does *not* happen when being manually invoked,
so it seems to be something related to the environment
of FAI. Couldn't spot the underlying issue yet though
and we clearly don't want to ship any such files
inside the root directory of the rootfs.