lang/scala: replace ensime with metals

Ensime just went missing from everywhere.
On [[ensime.github.io]], they encourage to switch to metals -
a new Scala language server already supported by lsp-mode.

This commit:
- removes ensime completely
- adds a doctor.el to check for metals-emacs binary

Original issue: https://discordapp.com/channels/406534637242810369/406554085794381833/628809956103028747
Metals: https://scalameta.org/metals/docs/editors/emacs.html

modules/lang/scala/config.el

@@ -15,23 +15,6 @@
     (add-hook 'scala-mode-local-vars-hook #'lsp!)))
 
 
-(use-package! ensime
-  :unless (featurep! +lsp)
-  :defer t
-  :config
-  (setq ensime-startup-snapshot-notification nil
-        ensime-startup-notification nil
-        ensime-eldoc-hints 'all
-        ;; let DOOM handle company setup
-        ensime-completion-style nil)
-
-  (set-company-backend! 'scala-mode '(ensime-company company-yasnippet))
-
-  ;; Fix void-variable imenu-auto-rescan error caused by `ensime--setup-imenu'
-  ;; trying to make imenu variables buffer local before imenu is loaded.
-  (require 'imenu))
-
-
 (use-package! sbt-mode
   :after scala-mode
   :config (set-repl-handler! 'scala-mode #'+scala/open-repl))