mu4e: clean up org-handling

Load org support by default, unless mu4e-org-support is set to nil.

Turn off speedbar support by default (set mu4e-speedbar-support to t to
re-enable it).

Move the non-obsolete org stuff to mu4e-org. Rename some things from
org-mu4e to mu4e-org.

Remove org-old-mu4e.el

mu4e/mu4e-vars.el

@@ -57,6 +57,16 @@ change until after quitting."
   :safe 'stringp
   :group 'mu4e)
 
+(defcustom mu4e-org-support t
+  "Support org-mode links."
+  :type 'boolean
+  :group 'mu4e)
+
+(defcustom mu4e-speedbar-support nil
+  "Support having a speedbar to navigate folders/bookmarks."
+  :type 'boolean
+  :group 'mu4e)
+
 (defcustom mu4e-get-mail-command "true"
   "Shell command to run to retrieve new mail.
 Common values are \"offlineimap\", \"fetchmail\" or \"mbsync\", but
@@ -70,6 +80,8 @@ already retrieved in another way."
   :group 'mu4e
   :safe 'stringp)
 
+
+
 (defcustom mu4e-index-update-error-warning t
   "Whether to display warnings during the retrieval process.
 This depends on the `mu4e-get-mail-command' exit code."