home: vim: do not use 'ftplugin' for 'null-ls'

Unfortunately, this registers the sources many times...

I fear that I will have to settle for a centralised configuration for
LSP-related options.

home/vim/after/ftplugin/haskell.vim

@@ -1,19 +1,6 @@
 " Create the `b:undo_ftplugin` variable if it doesn't exist
 call ftplugined#check_undo_ft()
 
-" Set-up LSP, linters, formatters
-lua << EOF
-local null_ls = require("null-ls")
-local utils = require("ambroisie.utils")
-
-null_ls.register({
-    null_ls.builtins.formatting.brittany.with({
-        -- Only used if available
-        condition = utils.is_executable_condition("brittany"),
-    }),
-})
-EOF
-
 " Use a small indentation value on Haskell files
 setlocal shiftwidth=2
 let b:undo_ftplugin.='|setlocal shiftwidth<'