Remove PurgeOldSecretsAsync methods

Remove `SecretManager.PurgeOldSecretsAsync()`. Some investigation may be needed to see when/why this behavior was removed, but it seems none of this code is used or needed anymore (including code in each Repository).

https://github.com/Azure/azure-functions-host/blob/d29280fc3978e0374211e21dcd4e29e4fcff109d/src/WebJobs.Script.WebHost/Security/KeyManagement/SecretManager.cs#L839

The logic won't work properly anyway -- it's not actually using function metadata to determine current functions. Instead, it's checking directories, which would lead to lots of problems should anything call this.

My guess is we can delete all this.

Start with the linked PurgeOldSecretsAsync method in src/WebJobs.Script.WebHost/Security/KeyManagement/SecretManager.cs, then search for its uses and inspect the corresponding Repository code. Done means the unused methods and related repository code are removed, with no remaining references and the host still building successfully.

Written by the indexing model from the issue text.