stack/slapos.cfg: remove slapos.extension.strip extension

see !1215
Status Job ID Name Coverage
  External
failed ERP5.StandaloneDeploymentScriptTest-Debian.Buster

failed ERP5.StandaloneDeploymentScriptTest-Debian.Stretch

failed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Buster

failed SlapOS-Master.StandaloneDeploymentScriptTest-Debian.Stretch

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Buster.Master

failed Wendelin.StandaloneDeploymentScriptTest-Debian.Stretch