We can delete the LimitRange resource with the subcommand delete. Like creating the LimitRange, deleting a LimitRange in a Namespace would remove the constraints in the Namespace automatically:
$ kubectl delete -f my-first-limitrange.yaml -n=my-namespace
limitrange "my-limitrange" deleted
$ kubectl describe ns my-namespace
Name: my-namespace
Labels: <none>
Annotations: <none>
Status: Active
No resource quota.
No resource limits.