cleanup legacy context after delete cluster

[RainbowMango]

What this PR does / why we need it:
Kube config context didn't be removed after delete clusters, that would block creating cluster next time as same context is already exist. You can reproduce it by following command.

# ./scripts/create-clusters.sh
# ./scripts/delete-clusters.sh
# kubectl config view
apiVersion: v1
clusters: null
contexts:
- context:
    cluster: kind-cluster1
    user: kind-cluster1
  name: cluster1
- context:
    cluster: kind-cluster2
    user: kind-cluster2
  name: cluster2
current-context: cluster1
kind: Config
preferences: {}
users: null

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:
With this patch, we can get a clean config as follows:

# kubectl config view
apiVersion: v1
clusters: null
contexts: null
current-context: ""
kind: Config
preferences: {}
users: null

[RainbowMango]

/assign @pmorie @hectorj2f

[RainbowMango]

If you want check what exactly the error is, see below:

[root@ecs-d8b6 kubefed]# ./scripts/delete-clusters.sh 
Deleting 2 clusters
Deleting cluster "cluster1" ...
Deleting cluster "cluster2" ...
Complete
[root@ecs-d8b6 kubefed]# ./scripts/create-clusters.sh 
Creating 2 clusters
Creating cluster "cluster1" ...
 ✓ Ensuring node image (kindest/node:v1.17.0) 🖼
 ✓ Preparing nodes 📦  
 ✓ Writing configuration 📜 
 ✓ Starting control-plane 🕹️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️️ 
 ✓ Installing CNI 🔌 
 ✓ Installing StorageClass 💾 
Set kubectl context to "kind-cluster1"
You can now use your cluster with:

kubectl cluster-info --context kind-cluster1

Thanks for using kind! 😊
Cluster "kind-cluster1" set.
error: cannot rename the context "kind-cluster1", the context "cluster1" already exists in /root/.kube/config

[hectorj2f]

I'd test it as soon as possible in my environment 👍.

[hectorj2f]

I tested and it works 👍
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hectorj2f, RainbowMango

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