Set-AuthConfig -ClearPreviousCertificate
Yea that doesnt remove the actual cert:
https://learn.microsoft.com/en-us/powershell/module/exchange/set-authconfig?view=exchange-ps
The ClearPreviousCertificate switch clears the certificate saved as the previous certificate in the authorization configuration.
If you renewed it and it looks good, you can delete the old one.