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/packages.el

@@ -3,9 +3,3 @@
 
 (package! sbt-mode)
 (package! scala-mode)
-
-(unless (featurep! +lsp)
-  ;; Fix #1697: Ensime doesn't have a master branch and its MELPA recipe doesn't
-  ;; specify a branch. Straight can't handle packages with non-standard primary
-  ;; branches (at the time of writing), so we must specify it manually:
-  (package! ensime :recipe (:branch "2.0")))