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NewLeaderElector: Remove nil k8s client check

Pulling this change out from my work on #3442 to simplify the PR.

There are two points that call NewLeaderElector: `benchmark/k8s.go` and
`benchmark/eks.go`. Both provide the `kubeClient` argument that is
received by `ClientProvider.GetClient`. Diggining in `GetClient`, it
ultimately comes from `kubernetes.GetKubernetesClient()` which only
returns a `nil` client when `err != nil` which means that
`NewLeaderElector` can never receive a nil client.

Removes the `DefaultUniqueManager` and simplifies the leader election logic.
The `NewLeaderElector` function will now always return a
`LeaderelectionManager` and will not fall back to a default manager.