Remove deprecated stream channel functionality

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.

Start by locating the deprecated multiplexers and the tests that reference them, then inspect HTTP2StreamChannel, MultiplexerAbstractChannel, HTTP2StreamMultiplexer, and the deprecated pipeline helpers. Use the checklist to trace each obsolete initializer and code path, and run the affected test suite and build checks after removal. Done means the deprecated functionality and its tests are gone without breaking the remaining HTTP/2 implementation.

Written by the indexing model from the issue text.