Revert "making :results replace the default"

This reverts commit a13cbf64b6462cf007632c68bf6cf8fc298e919a.

I'm in favour of this change, but it seems that we need a more sophisticated way of combining plists before we can
change the default header args in this way. Otherwise, we get two entries for :results (one from the defaults and one
from the header args), whereas what we want is a single entry for :results with space-separated values.

lisp/org-babel.el

@@ -49,7 +49,7 @@ then run `org-babel-pop-to-session'."
 
 (add-hook 'org-metadown-hook 'org-babel-pop-to-session-maybe)
 
-(defvar org-babel-default-header-args '((:session . "none") (:results . "replace"))
+(defvar org-babel-default-header-args '((:session . "none"))
   "Default arguments to use when evaluating a source block.")
 
 (defvar org-babel-default-inline-header-args '((:results . "silent") (:exports . "results"))