Cats-effect: Consider whether LinkedMap is still necessary

[RaasAhsan]

As part of https://github.com/typelevel/cats-effect/pull/918, Deferred and MVarConcurrent were refactored so that callbacks are uniquely identified by a monotonically increasing long. This is in fact how LinkedMap tracks insertion order, so we may not actually need it anymore. If that's the case, we should remove it both on the 2.x series and the 3.x series (after https://github.com/typelevel/cats-effect/issues/942).

[djspiewak]

This is an absolutely lovely observation, and it seems likely to be true!

[SystemFw]

I'm on this btw