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: 6288099561
Metals: https://scalameta.org/metals/docs/editors/emacs.html

modules/lang/scala/doctor.el

@@ -0,0 +1,9 @@
+;;; lang/scala/doctor.el -*- lexical-binding: t; -*-
+
+(assert! (or (not (featurep! +lsp))
+             (featurep! :tools lsp))
+         "This module requires (:tools lsp)")
+
+(if (and (featurep! +lsp)
+         (not (executable-find "metals-emacs")))
+    (warn! "metals-emacs isn't installed"))