Following up from #159886, we should remove dartHostTestHandler entirely from Pigeon:

• It's an extra generator we have to maintain.
• It relies on special test hooks for method channels, which means it won't work if we transition Pigeon to using FFI internally.
• In every case where we were using it, switching to an injected (or in one case, global wrapper) instance did not make testing any harder.
• Having individual packages unit test parts of the generated code (which that structure did) is not useful now that we have extensive testing of Pigeon itself.

The plan is to deprecate it, wait a while to see if there is any feedback highlighting a case we never encountered where it provides significant value in writing tests, and if not, remove it.