Don't re-export `Alternative(..)` from Control.Monad (re #9586)

This was done in d94de87252d0fe2ae97341d186b03a2fbe136b04 to avoid orphans
but since a94dc4c3067c6a0925e2e39f35ef0930771535f1 moved `Alternative`
into GHC.Base, this isn't needed anymore.

This is important, as otherwise this would require a non-neglectable amount
of `Control.Monad hiding ((<|>), empty)` imports in user code.

The Haddock submodule is updated as well

Test Plan: partial local ./validate --fast, let Harbormaster doublecheck it

Reviewed By: ekmett, austin

Differential Revision: https://phabricator.haskell.org/D248