modify kustomization.yaml

[binoue]

Metallb is now using kustomize to generate Kubernetes resource files since v0.8.3

I noticed that in the manifest directory both namespace.yaml and metallb.yaml contain the same namespace resource, but metallb.yaml does not need the namespace resource if kustomization.yaml merge both files.

It does not only reduce codes but also beneficial for the users who want to use metallb resources in a different namespace.
(They do not need to edit the upsteam yaml file and only need to edit kustomization.yaml)

Therefore, this pull request made the following changes.

• delete namespace resource from metallb.yaml
• modify kustomization.yaml to merge metallb.yaml and namespace.yaml

[danderson]

Thanks!