Revert "Use nested virt in scenario jobs"

Nested virt doesn't work well on infra nodes.
This reverts commit 190755866c6dbbd735dcae3ee9434c8f03722763.

Change-Id: I43eeb502ba07cf5745a17015df7fdc7120df6eaa
Closes-Bug: #1788006
diff --git a/.zuul.yaml b/.zuul.yaml
index 1f8c71e..a2506c7 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -121,7 +121,6 @@
         DEFAULT_INSTANCE_TYPE: ds512M
         DEFAULT_INSTANCE_USER: ubuntu
         BUILD_TIMEOUT: 784
-        LIBVIRT_TYPE: kvm
       devstack_services:
         cinder: true