rcutorture: Remove obsolete kversion param in kvm.sh

Now that we have removed configs based on kernel version, we can also remove the
kversion parameter in kvm.sh.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>