mu4e: remove mu4e-use-maildirs and defer loading of package

To deactivate maildirs extension, simply add the package to
dotspacemacs-excluded-packages

layers/+email/mu4e/README.org

@@ -42,13 +42,8 @@ The maildirs extension adds a list of all your maildirs to the main mu4e view
 that by default shows the unread and total mail counts for all your mail under
 your base mail directory.
 
-If you wish enable the maildirs extension you can set =mu4e-use-maildirs= to non
-nil value.
-
-#+begin_src emacs-lisp
-  (setq-default dotspacemacs-configuration-layers
-                '((mu4e :variables mu4e-use-maildirs t)))
-#+end_src
+This extension is enabled by default, you can deactivate (and uninstall) it by
+adding the package == to your dotfile variable =dotspacemacs-excluded-packages=.
 
 ** Multiple Accounts
 This layer includes support for multiple sending accounts.

layers/+email/mu4e/config.el

@@ -16,8 +16,5 @@
 (defvar mu4e-account-alist nil
   "Account alist for custom multi-account compose.")
 
-(defvar mu4e-use-maildirs nil
-  "Set to non-nil to use the maildirs extension.")
-
 (when mu4e-installation-path
   (push mu4e-installation-path load-path))