Remove some references to Emacs 21

* lisp/erc/erc-track.el (erc-track-position-in-mode-line):
* lisp/erc/erc.el (erc-header-line-format):
* lisp/ibuffer.el (ibuffer-mode):
* lisp/ruler-mode.el: Remove some references to Emacs 21.

lisp/erc/erc-track.el

@@ -244,8 +244,6 @@ The effect may be disabled by setting this variable to nil."
 (defcustom erc-track-position-in-mode-line 'before-modes
   "Where to show modified channel information in the mode-line.
 
-Setting this variable only has effect in GNU Emacs versions above 21.3.
-
 Choices are:
 `before-modes' - add to the beginning of `mode-line-modes',
 `after-modes'  - add to the end of `mode-line-modes',

lisp/erc/erc.el

@@ -6290,7 +6290,6 @@ The following characters are replaced:
 
 (defcustom erc-header-line-format "%n on %t (%m,%l) %o"
   "A string to be formatted and shown in the header-line in `erc-mode'.
-Only used starting in Emacs 21.
 
 Set this to nil if you do not want the header line to be
 displayed.

lisp/ibuffer.el

@@ -2579,7 +2579,7 @@ will be inserted before the group at point."
   (setq buffer-read-only t)
   (buffer-disable-undo)
   (setq truncate-lines ibuffer-truncate-lines)
-  ;; This makes things less ugly for Emacs 21 users with a non-nil
+  ;; This makes things less ugly for users with a non-nil
   ;; `show-trailing-whitespace'.
   (setq show-trailing-whitespace nil)
   ;; disable `show-paren-mode' buffer-locally

lisp/ruler-mode.el

@@ -25,7 +25,7 @@
 ;;; Commentary:
 
 ;; This library provides a minor mode to display a ruler in the header
-;; line.  It works from Emacs 21 onwards.
+;; line.
 ;;
 ;; You can use the mouse to change the `fill-column' `comment-column',
 ;; `goal-column', `window-margins' and `tab-stop-list' settings: