Close the GHCJS support window?

Related to #11608.

We have two very similar modules.

- `Cabal/src/Distribution/Simple/GHC.hs`
- `Cabal/src/Distribution/Simple/GHCJS.hs`

If the GHCJS module is unmaintained can we remove it?

> @philderbeast Do you think it's worth it to update this codepath? The GHCJS codepath is untested and unmaintained, I think the last GHCJS release was corresponding to 8.10 and the modern way is to use the javascript backend. I couldn't be confident that anyway I did change it was correct.

_Originally posted by @mpickering in https://github.com/haskell/cabal/pull/11450#discussion_r2924003360_

> ghcjs is going away I think, so may be this should be closed?..

_Originally posted by @ulysses4ever in [#5844](https://github.com/haskell/cabal/issues/5844#issuecomment-4020467773)_

> The default branch, the [`ghc-8.10` branch](https://github.com/ghcjs/ghcjs/commits/ghc-8.10/), of the GHCJS fork was last updated Sep 25, 2022.

_Originally posted by @philderbeast in https://github.com/haskell/cabal/issues/11530#issuecomment-4046295676_

Start by comparing Cabal/src/Distribution/Simple/GHC.hs and Cabal/src/Distribution/Simple/GHCJS.hs, then inspect references to the GHCJS module. Confirm whether the GHCJS codepath is still supported before removing it; done means obsolete references are addressed and the project remains consistent.

Written by the indexing model from the issue text.