Temporary revert GPG feature for semver based retagging (#373)

Feature #343 happens to be a breaking change when enabling `.Values.signing` but not specifying
any of the new private key properties. Tag `v6.0.2` is therefore not following semantic versioning.

This temporarily reverts commit b8f0310c43 and a fix-up commit 57a1cd27d9
to retag 6.0.2 as 6.0.3.

Co-authored-by: justusbunsi <sk.bunsenbrenner@gmail.com>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/373
Reviewed-by: techknowlogick <techknowlogick@gitea.io>
Reviewed-by: luhahn <luhahn@noreply.gitea.io>

CONTRIBUTING.md

@@ -50,13 +50,3 @@ be used:
    forwarded first from `minikube` to localhost first via `kubectl --namespace
    default port-forward svc/gitea-http 3000:3000`. Now Gitea is accessible at
    [http://localhost:3000](http://localhost:3000).
-
-### Unit tests
-
-```bash
-# install the unittest plugin
-$ helm plugin install https://github.com/heyhabito/helm-unittest
-
-# run the unittests
-make unittests
-```