Replace `time-to-seconds´ with `org-float-time´

* lisp/org.el (org-current-time): Replace call to obsolete function
  `time-to-seconds´ with a call to compatibility function
  `org-float-time´.

XEmacs 21.5 already has float-time, so we may not need a compatibility
function here depending on what version of XEmacs introduced it and
what is the earliest version that Org is supposed to still support.

lisp/org.el

@@ -5338,8 +5338,8 @@ the rounding returns a past time."
 	    (apply 'encode-time
 		   (append (list 0 (* r (floor (+ .5 (/ (float (nth 1 time)) r)))))
 			   (nthcdr 2 time))))
-      (if (and past (< (time-to-seconds (time-subtract (current-time) res)) 0))
-	  (seconds-to-time (- (time-to-seconds res) (* r 60)))
+      (if (and past (< (org-float-time (time-subtract (current-time) res)) 0))
+	  (seconds-to-time (- (org-float-time res) (* r 60)))
 	res))))
 
 (defun org-today ()