Revert part of "Add base interface for IHubContext (#33443)"

[BrennanConroy]

Original change #33443

The change is a breaking change:

❌ DISALLOWED: Adding an interface to the set of base types of an interface

Apps rolling forward from a 5.0 runtime to a 6.0 runtime would break (if using SignalR obviously)

Keeping the out THub change as it isn't a binary breaking change I believe. It can be a behavior breaking change in extremely rare scenarios that no one is doing probably.

class BaseHub : Hub
{}

class DerivedHub : BaseHub
{}

if (x is IHubContext<BaseHub>)
{
    // now matches for IHubContext<DerivedHub>
}
else if (x is IHubContext<DerivedHub>)
{
}

[davidfowl]

Keep the non generic interface and implement it on the concrete type so the original scenario is still possible.