Revert "* lisp/subr.el (when): Use `macroexp-progn'"

This reverts commit 8e843831ea.
It breaks bootstrapping (duh).

lisp/subr.el

@@ -179,7 +179,7 @@ value of last one, or nil if there are none.
 
 \(fn COND BODY...)"
   (declare (indent 1) (debug t))
-  (list 'if cond (macroexp-progn body)))
+  (list 'if cond (cons 'progn body)))
 
 (defmacro unless (cond &rest body)
   "If COND yields nil, do BODY, else return nil.