K8s secrets for repository credential templates are not deleted when credential template is deleted

[jannfis]

Checklist:

• I've searched in the docs and FAQ for my answer: http://bit.ly/argocd-faq.
• I've included steps to reproduce the bug.
• I've pasted the output of argocd version.

Describe the bug

The K8s secret that is created to store the secrets used for repository credentials are not deleted when the repository credential is deleted, thus leaving orphaned K8s secrets behind.

To Reproduce

• Create repository credential template, i.e. argocd repocreds add https://yourserver/path --ssh-private-key-path ~/.ssh/id_rsa
• A corresponding K8s secret is created in argocd namespace
• Delete the credential template created in first step, i.e. argocd repocreds rm https://yourserver/path
• Check that previously created K8s secret still exist in cluster

Expected behavior

The corresponding K8s secret should be deleted when the credential template is removed

Screenshots

If applicable, add screenshots to help explain your problem.

Version

argocd: v1.4.0+b28bdb7.dirty
  BuildDate: 2020-01-22T10:10:27Z
  GitCommit: b28bdb7f4dc971f1d4f28cd28546aba2ed09c0a6
  GitTreeState: dirty
  GoVersion: go1.12.5
  Compiler: gc
  Platform: linux/amd64
argocd-server: v99.99.99+unknown
  BuildDate: 1970-01-01T00:00:00Z
  GitCommit: 
  GitTreeState: 
  GoVersion: go1.12.5
  Compiler: gc
  Platform: linux/amd64
  Ksonnet Version: vdev-2019-05-15T19:48:06+0200
  Kustomize Version: Version: {KustomizeVersion:3.1.0 GitCommit:95f3303493fdea243ae83b767978092396169baf BuildDate:2019-07-26T18:11:16Z GoOs:linux GoArch:amd64}
  Helm Version: v2.13.1
  Kubectl Version: v1.15.2