Drop `hls-retrie-plugin`

[fendor]

retrie is abandoned https://github.com/haskell/haskell-language-server.git, so the days of the plugins are numbered. It also isn't supported in any recent GHC versions.

Development continues in https://github.com/xich/retrie, but this fork also doesn't support the more recent GHC versions.