Clean up certmanager conformance tests

[skonto]

What version of Knative?

1.14

Expected Behavior

Certmanager conformance tests should be removed as they are tested already as part of the regular
Serving tests and also they simply test the secret creation for a certificate request, they are not end-to-end tests.

Actual Behavior

Tests should be removed and all signers should be tested along with the other Serving tests

Steps to Reproduce the Problem

n/A

[skonto]

I am working on this here: #15261
I will remove the tests from the infra repo too.