re6stnet: drop backward compatibility code

All instances in the wild have now moved their DB
from srv/res6stnet/registry to srv/re6stnet.

See commit a1ed9c30.
4 jobs for 1.0
Status Job ID Name Coverage
  External
failed Cloudooo.UnitTest-Master

canceled SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

passed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master