Refactor and/or remove dead code in channel factory async close path

PR #1071 made the channel factory async close path remain async. This appears to have made the synchronous code path unreachable, because the low level IAsyncCommunicationObject logic uses the async close path even for sync closes

To resolve this issue:
- Establish via code coverage and analysis that the TypedServiceChannelFactory and ServiceChannelFactory OnClose and BeginClose have become unreachable
- Remove what can be removed or refactor to use common code path