Remove `kubernetes: active` pipeline build policy

Description

We still support only/except: kubernetes: active pipeline build policy, that is still marked as ALPHA feature in docs ➡ https://docs.gitlab.com/ee/ci/yaml/#only-and-except-complex

From Slack:

Kamil: @grzesiek can we deprecate kubernetes: active given that we have variables: $KUBECONFIG today? also I’m not sure if this works OK any longer.

I think it works, but I'm fine with removing it because it simplify codebase and we do have a little more idiomatic way of doing that.

This feature is still marked as alpha feature, so we can perhaps remove it without deprecation period.

Are we still using it in autodevops / templates somewhere?

/cc @jlenny @ayufan @DylanGriffith