Remove nodeSelector for flannel DaemonSet

According to upstream kube-flannel.yml PR[1], node-selector introduced
is because flannel image doesn't support multiarch manifestes. Which
means that it can't specify flannel:version-arch images in the same
daemonset for every arch platform. To make every arch platform can
deploy flannel upstream add one daemonset with nodeSelector per arch.

But in magnum flannel image tag is configurable via label, thus every
arch platform can use one daemonset to deploy by specify corresponding
flannel image tag. So nodeSelector is unnecessary here.

[1]: https://github.com/coreos/flannel/pull/989

Change-Id: I97e78e8d77973e03eeff598b212287945ca00190
Task: 39453
Story: 2007026

magnum/drivers/common/templates/kubernetes/fragments/flannel-service.sh

@@ -159,7 +159,7 @@ data:
 apiVersion: apps/v1
 kind: DaemonSet
 metadata:
-  name: kube-flannel-ds-${ARCH}
+  name: kube-flannel-ds
   namespace: kube-system
   labels:
     tier: node
@@ -181,8 +181,6 @@ spec:
         seLinuxOptions:
           type: "spc_t"
       hostNetwork: true
-      nodeSelector:
-        beta.kubernetes.io/arch: ${ARCH}
       tolerations:
       - operator: Exists
         effect: NoSchedule