grilo: Do not run on OSTree images

The grilo test installs 2 packages as dependencies. On OSTree, apt is
not available, so the test cannot be run on these images.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>

test-cases/grilo.yaml

@@ -7,7 +7,6 @@ metadata:
     sdk:     [ amd64 ]
   image-deployment:
     - APT
-    - OSTree
   group: "common"
   type: functional
   exec-type: automated