Remove Kubernetes External Secret from docs

It seems to be deprecated.
https://github.com/external-secrets/kubernetes-external-secrets

Signed-off-by: kondo takeshi <chaspy@users.noreply.github.com>

docs/operator-manual/secret-management.md

@@ -3,7 +3,6 @@
 Argo CD is un-opinionated about how secrets are managed. There's many ways to do it and there's no one-size-fits-all solution. Here's some ways people are doing GitOps secrets:
 
 * [Bitnami Sealed Secrets](https://github.com/bitnami-labs/sealed-secrets)
-* [GoDaddy Kubernetes External Secrets](https://github.com/godaddy/kubernetes-external-secrets)
 * [External Secrets Operator](https://github.com/external-secrets/external-secrets)
 * [Hashicorp Vault](https://www.vaultproject.io)
 * [Banzai Cloud Bank-Vaults](https://github.com/banzaicloud/bank-vaults)