remove namespace field from secret reference in AccessRequest status

[Diaphteiros]

What this PR does / why we need it:
Removes the superfluous namespace field from the secret reference in the AccessRequest status. AccessRequest secrets are expected to be created in the same namespace as the AccessRequest itself, so this field is not needed. It was added by accident when we unified the reference types.

Which issue(s) this PR fixes:
Fixes #103

Special notes for your reviewer:
⚠️ This is a breaking change. While it should have no impact regarding existing resources, because all currently existing ClusterProviders create the AccessRequest secrets in the same namespace as the AccessRequest, all controllers that read the secret reference will require minor code changes.

Let's choose a good point in time to merge this.

Release note:

Removed the `status.secretRef.namespace` field from `AccessRequest` resources which was added by accident. The access secrets are expected to be in the same namespace as the AccessRequest itself, so wherever this field is read, it can just be replaced with the AccessRequest's namespace.