Remove `MonadCont` instance

[garyb]

Since we don't want to encourage continuation-style behaviour for Aff

[garyb]

Doh, I thought we might have wanted to include this in 1.0.0... or not include should I say.

[jdegoes]

I'd prefer to deprecate it for a number of releases. It's not really harming anything, AFAIK.

[garyb]

Oh ok, great! I had the impression that it might have been broken (slightly) by some of the recent changes.