Remove state machine allocation from HttpConnection.ReadAsync

[MihaZupan]

This one was async solely to log a debug-level event.

Used by most of our H1 response content streams.

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[stephentoub]

On quick glance it doesn't look like an issue, but it'd be good to confirm there aren't any expected exceptions that might now escape synchronously into user code from a Task-returning method.

[MihaZupan]

All our callers for this one are async at some point.