[reciever/k8s_cluster] Remove redundant `opencensus.resourcetype` resource attribute

[dmitryax]

Component(s)

receiver/k8scluster

Is your feature request related to a problem? Please describe.

This attribute doesn't bring any value. It was introduced as a side effect of using OpenCensus metrics model to collect the metrics.

Describe the solution you'd like

Given that the receiver is already migrated to pdata and doesn't use OC model anymore, we can remove the attribute. Ideally, it should be done through a deprecation process, which requires #19174 to be resolved first.

[povilasv]

I can work on this, once mdatagen change lands