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-icalendar.el

@@ -39,7 +39,6 @@
 
 ;;; Code:
 
-(require 'mu4e)
 (require 'gnus-icalendar)
 (require 'cl-lib)