refactor(elm): remove tree-sitter support

No *-ts-mode exists for elm-mode. The elm module will fall back to
font-lock rules so this is not a breaking changing.

modules/lang/elm/README.org

@@ -13,9 +13,6 @@ This module adds [[https://elm-lang.org/][Elm]] support to Doom Emacs.
 - +lsp ::
   Enable LSP support for ~elm-mode~. Requires [[doom-module::tools lsp]] and a langserver
   (supports [[https://github.com/elm-tooling/elm-language-server][elm-language-server]]).
-- +tree-sitter ::
-  Leverages tree-sitter for better syntax highlighting and structural text
-  editing. Requires [[doom-module::tools tree-sitter]].
 
 ** Packages
 - [[doom-package:elm-mode]]

modules/lang/elm/config.el

@@ -5,9 +5,6 @@
       (add-hook 'elm-mode-local-vars-hook #'lsp! 'append)
     (set-company-backend! 'elm-mode 'company-elm))
 
-  (when (modulep! +tree-sitter)
-    (add-hook 'elm-mode-local-vars-hook #'tree-sitter! 'append))
- 
   (set-repl-handler! 'elm-mode #'run-elm-interactive)
   (set-ligatures! 'elm-mode
     :null "null"

modules/lang/elm/doctor.el

@@ -1,5 +1,2 @@
 ;;; lang/elm/doctor.el -*- lexical-binding: t; -*-
 
-(assert! (or (not (modulep! +tree-sitter))
-             (modulep! :tools tree-sitter))
-         "This module requires (:tools tree-sitter)")