rem: nil: Continue removal of license headers from test zones

Copyright license headers were removed from system test zone files in
commit f144db6b686, but this change only applied to files named '*.db',
'*.db.in', etc. There were some zone files called '*.zone' which were
left unchanged; these have been updated now as well.

Merge branch 'each-cleanup-zonefiles' into 'main'

See merge request isc-projects/bind9!11853