use default k8s version for spot e2e test

It looks like there was some copy-pasta when this test was first made
that locked it to the Flatcar test's k8s version which seems like a
mistake. This PR removes that to fallback to the default to fix this
test.

Author: nojnhuh

test/e2e/azure_test.go

@@ -415,7 +415,6 @@ var _ = Describe("Workload cluster creation", func() {
 				withFlavor("spot"),
 				withNamespace(namespace.Name),
 				withClusterName(clusterName),
-				withKubernetesVersion(e2eConfig.GetVariable(FlatcarKubernetesVersion)),
 				withControlPlaneMachineCount(1),
 				withWorkerMachineCount(1),
 				withControlPlaneWaiters(clusterctl.ControlPlaneWaiters{