Remove app-reg ar-radix-networkpolicy-canary

[nilsgstrabo]

This app-registration is used in oauth2 config for app radix-networkpolicy-canary, which is used by cicd-canary to test egress network policies.
These tests does not need a valid app-reg and client secret, since they only test if the oauth2 service can be reached (it does not test that the oauth2 flow works). We can therefore delete the app-registration, corresponding secrets in key-vaults, related terraform in radix-platform, and docs in radix-private