[Elixir] Remove traces of flycheck-mix from docs and layer variables

flycheck-mix has been removed by the package author as being
obsolete with proper lsp and alchemist support. Therefore
the layer also has been cleared of it as it cannot longer
be found on Melpa.

layers/+lang/elixir/README.org

@@ -15,7 +15,6 @@
   - [[#flycheck][Flycheck]]
     - [[#credo][Credo]]
     - [[#dogma][Dogma]]
-    - [[#mix-compile][mix compile]]
   - [[#language-server-protocol][Language server protocol]]
   - [[#debugger][Debugger]]
 - [[#key-bindings][Key bindings]]
@@ -55,7 +54,6 @@ As Alchemist is no longer maintained, elixir-ls is a preferred solution, even th
 - Smart code completion
 - Elixir project management
 - Integration with [[http://company-mode.github.io/][company-mode]]
-- Flycheck support for mix compile
 - Flycheck support for [[https://github.com/rrrene/credo][credo]]
 - Flycheck support for test results
 - Interactive debugger using [[https://github.com/emacs-lsp/dap-mode][dap-mode]]
@@ -152,38 +150,6 @@ Then run in your shell:
   $ mix deps.get
 #+END_SRC
 
-*** mix compile
-*Important:*
-
-Elixir compiler is based on macros and can execute arbitrary during compilation.
-Therefore Spacemacs disable flycheck compilation checker by default.
-
-To enable flycheck support for compilation errors *globally* the variable
-=elixir-enable-compilation-checking= can be set explicitly to =t= in your
-dotfile but it is not recommended to do so because of the limitation described
-above.
-
-Instead you should use directory local variables in order to enable the flycheck
-checker only for certain projects. Directory local variables are stored in a
-file named =.dir-locals.el= usually at the root of a project. To easily add a
-directory local variable use the key binding ~SPC f v d~ then choose the
-=elixir-mode= and the variable name =elixir-enable-compilation-checking= with a
-value of t. The result is a new file =.dir-locals.el= with the following
-contents:
-
-#+BEGIN_SRC elisp
-  ;;; Directory Local Variables
-  ;;; For more information see (info "(emacs) Directory Variables")
-
-  ((elixir-mode
-    (elixir-enable-compilation-checking . t)))
-#+END_SRC
-
-Spacemacs marks the variable =elixir-enable-compilation-checking= as safe so
-Emacs won't ask you if the variable is safe whenever an elixir file is opened.
-
-Remember that you can verify the flycheck checkers status with ~SPC e v~.
-
 ** Language server protocol
 The =lsp= backend uses [[https://github.com/JakeBecker/elixir-ls][elixir-ls]] as its language server implementation
 Clone the project to your system and follow the building instructions [[https://github.com/JakeBecker/elixir-ls#building-and-running][here]]

layers/+lang/elixir/config.el

@@ -11,12 +11,6 @@
 
 ;; Variables
 
-(defvar elixir-enable-compilation-checking nil
-  "If non-nil syntax checking is enable for compilation.
-Default is nil because Elixir compilation is based on macros and thus it
-is unsafe. Activate this option only for trusted code, usage of a
-directory variable is recommended.")
-
 (defvar elixir-backend nil
   "The backend to use for IDE features.
 Possible values are `alchemist' and `lsp'.