Remove use of IConfiguration from ServiceCollection

[halter73]

AddMvc, AddSignalR, GetDefaultServices, etc... already take in an IConfiguration object and use a ServiceDescriber to pull in replacement service implementations from config.

If you were to create your own ServiceCollection (which is atypical) and pass in an IConfiguration object to the constructor, it would not be respected by AddMvc and family.

For these reasons, we should remove the constructor from ServiceCollection that accepts an IConfiguration object, and stop using ServiceDescriber in ServiceCollection since it doesn't provide as much utility when an IConfiguration object is provided.