Removing the non-asyncio reactor support

I think it's time to at least open this discussion.

Pros: removes a lot of compatibility code and poorly tested alternative code paths from Scrapy, simplifies the docs. Allows us to deprecate TelnetConsole in favor of aioconsole and/or ipykernel. Cons: losing possible performance benefits? Some Windows/asyncio/3.14+ incompatibilities? Losing some platform-specific reactors that are based on different polling mechanisms than AsyncioSelectorReactor?