Remove -XMonadFailDesugaring as per MFP

Summary

-XMonadFailDesugaring should have been removed in 8.8 as per MFP#Transition.

Steps to reproduce

Enable -XMonadFailDesugaring: it's accepted (having no effect).

Expected behavior

The extension is rejected -- as unknown or removed (not sure which one?).

Environment

GHC version used: GHC HEAD, 8.10

Discussion

This is a follow-up on the discussion over ghc-steering-committee. Quoting @simonpj:

We were due to remove them in 8.8, as the proposal says under "Transitional Strategy" https://gitlab.haskell.org/haskell/prime/-/wikis/libraries/proposals/monad-fail But we failed to do so. Would someone like to make a ticket to do them?

What bothers me a bit is the User Guide's wording:

This extension is enabled by default since GHC 8.6.1, under the MonadFail Proposal (MFP).

This extension is temporary, and will be deprecated in a future release.

This sounds much far from removal than it's from MFP's plan.

Also, there's another action item there, which, probably deserves its own issue:

Control.Monad.Fail is now a redundant module that can be considered deprecated.

Edited Jan 05, 2021 by Artem Pelenitsyn

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