ob-core: do not declare and define org-babel-call-process-region-original

* lisp/ob-core.el (org-babel-call-process-region-original): Change
  declaration into definition with nil initial value at the beginning
  of the file and drop the later definition.  Add comment that the
  dynamic scoping of this variable is done for tramp.

lisp/ob-core.el

@@ -33,7 +33,7 @@
   (if (memq system-type '(windows-nt cygwin))
       ".exe"
     nil))
-(defvar org-babel-call-process-region-original)
+(defvar org-babel-call-process-region-original nil) ;; dynamically scoped for tramp
 (defvar org-src-lang-modes)
 (defvar org-babel-library-of-babel)
 (declare-function show-all "outline" ())
@@ -2527,7 +2527,6 @@ of the string."
                     (org-babel-chomp (org-reverse-string string) regexp))
                    regexp))
 
-(defvar org-babel-org-babel-call-process-region-original nil)
 (defun org-babel-tramp-handle-call-process-region
   (start end program &optional delete buffer display &rest args)
   "Use Tramp to handle `call-process-region'.