Remove Brittany Plugin

[fendor]

Brittany is seemingly semi-official end of life: https://github.com/lspitzner/brittany

Should we remove the plugin?

[santiweight]

How popular is Brittany? Unfortunately it wasn't in the state of Haskell survey.

[fendor]

Doesn't quite matter, in our own definition, Brittany is a tier 3 plugin https://github.com/haskell/haskell-language-server/blob/master/docs/support/plugin-support.md with an abandoned base project.

Maybe after the next release, we should drop Brittany, since it will support almost none of the GHC versions we do support.

[andys8]

For reference: lspitzner/brittany#377 ("Announce end of maintenance")