Remove deprecated stream channel functionality

[Lukasa]

In #214 we deprecated and replaced the old stream channel. When we do a 2.0.0 release, we should remove the old code paths, reducing binary size, compile time, and resolving some minor performance problems.

• Remove the deprecated multiplexers and all tests that reference them.
• Remove the genericism from HTTP2StreamChannel, as it’s no longer necessary.
• Remove the extra code paths from MultiplexerAbstractChannel: it can concretely hold a single HTTP2StreamChannel
• Remove the initializers and code paths in HTTP2StreamMultiplexer that are deprecated or no-longer called
• Remove all deprecated pipeline helpers.