armv8r64-extras/kas: Use kirkstone branch for meta-virtualization

The kirkstone branch has now been created in meta-virtualization, so
remove the refspec master override in virtualization.yml

Issue-Id: SCM-4386
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: Id70f0d94fd4f8540e930f15a31aa8d0c7cf8c7de