Remove mmm-mode support as it has side effects in markdown-mode

this mode seems to be misconfigured and is
killing markdown-mode font handling.

As I did not noticed a lost of functionality
I have removed it from now.

If anyone is willing to reconfigure the mode
I am open to add it back.

Fixes syl20bnr#16170

layers/+lang/markdown/README.org

@@ -11,7 +11,6 @@
 - [[#install][Install]]
 - [[#configuration][Configuration]]
   - [[#live-preview][Live preview]]
-  - [[#automatic-mmm-mode-generation][Automatic MMM-Mode Generation]]
 - [[#usage][Usage]]
   - [[#generate-a-toc][Generate a TOC]]
 - [[#key-bindings][Key bindings]]
@@ -89,21 +88,6 @@ Additionally there is =flymd= package in =chrome= layer that allows to preview
 markdown buffer in Chrome. Please refer to =chrome= layer documentation for more
 information.
 
-** Automatic MMM-Mode Generation
-To generate MMM-Modes for languages set the value of the variable
-=markdown-mmm-auto-modes= to a list of the languages. For languages where the
-mode name directly relates to the language name, use a string. Otherwise, use a
-list of `("language" "mode")`:
-
-#+BEGIN_SRC emacs-lisp
-  dotspacemacs-configuration-layers '(
-    (markdown :variables markdown-mmm-auto-modes '("c" "c++" "python" "scala" ("elisp" "emacs-lisp"))))
-#+END_SRC
-
-*Note:* Spacemacs already defines the variable =markdown-mmm-auto-modes= to a
-bunch of languages. Consider opening a PR on the Spacemacs repository to add
-new languages instead of overriding the variable in your dotfile.
-
 * Usage
 ** Generate a TOC
 To generate a table of contents type on top of the buffer:

layers/+lang/markdown/config.el

@@ -26,26 +26,5 @@
 (defvar markdown-live-preview-engine 'eww
   "Possibe values are `eww' (built-in browser) or `vmd' (installed with `npm').")
 
-(defvar markdown-mmm-auto-modes
-  '(
-    ;; in alphabetical order, symbols first then lists
-    "c"
-    "c++"
-    "css"
-    "java"
-    "javascript"
-    "python"
-    "ruby"
-    "rust"
-    "scala"
-
-    ("elisp" "emacs-lisp")
-    ("ess" "R")
-    ("ini" "conf-unix")
-    ("html" "web")
-    )
-  "List of language names or lists of language and mode names for which to
-generate mmm classes.")
-
 (defvar markdown--key-bindings-modes '(markdown-mode gfm-mode)
   "Modes using markdown key bindings.")

layers/+lang/markdown/funcs.el

@@ -21,11 +21,6 @@
 ;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 
-(defun spacemacs/activate-mmm-mode ()
-  (unless (bound-and-true-p git-commit-mode)
-    ;; Enable `mmm-mode'.
-    (mmm-mode 1)))
-
 ;; Insert key for org-mode and markdown a la C-h k
 ;; Based on SE https://emacs.stackexchange.com/a/2208
 (defun spacemacs/insert-keybinding-markdown (key)
@@ -39,21 +34,6 @@ Will work on both org-mode and any mode that accepts plain html."
       (insert (format tag ""))
       (forward-char -6))))
 
-;; from Jason Blevins http://jblevins.org/log/mmm
-(defun markdown/mmm-auto-class (lang)
-  (let* ((l (if (listp lang) (car lang) lang))
-         (s (if (listp lang) (cadr lang) lang))
-         (class (intern (concat "markdown-" l)))
-         (submode (intern (concat s "-mode")))
-         (front (concat "^```" l "[\n\r]+"))
-         (back "^```$"))
-    (mmm-add-classes (list (list class
-                                 :submode submode
-                                 :front front
-                                 :back back)))
-    (dolist (mode markdown--key-bindings-modes)
-      (mmm-add-mode-ext-class mode nil class))))
-
 (defun spacemacs//markdown-hjkl-promotion-demotion (style)
   "Set promotion/demotiion on 'hjkl' for the given editing STYLE."
   (when (or (eq 'vim style)

layers/+lang/markdown/packages.el

@@ -29,7 +29,6 @@
     gh-md
     markdown-mode
     markdown-toc
-    mmm-mode
     smartparens
     valign
     (vmd-mode :toggle (eq 'vmd markdown-live-preview-engine))))
@@ -195,15 +194,6 @@
       (spacemacs/set-leader-keys-for-major-mode mode
         "it" 'markdown-toc-generate-toc))))
 
-(defun markdown/init-mmm-mode ()
-  (use-package mmm-mode
-    :commands mmm-mode
-    :init (add-hook 'markdown-mode-hook 'spacemacs/activate-mmm-mode)
-    ;; Automatically add mmm class for languages
-    :config
-    (mapc 'markdown/mmm-auto-class markdown-mmm-auto-modes)
-    (spacemacs|hide-lighter mmm-mode)))
-
 (defun markdown/init-vmd-mode ()
   (use-package vmd-mode
     :defer t