org.el: Drop subtract-time -> time-subtract alias

time-subtract has been available since Emacs 22.1.

lisp/org.el

@@ -4798,7 +4798,6 @@ This is for getting out of special buffers like capture.")
 
 ;; Other stuff we need.
 (require 'time-date)
-(unless (fboundp 'time-subtract) (defalias 'time-subtract 'subtract-time))
 (when (< emacs-major-version 28)  ; preloaded in Emacs 28
   (require 'easymenu))