org-agenda.el: don't prevent the use of M-x text-scale-mode RET

* lisp/org-agenda.el (org-agenda-mode): Fix bug: don't prevent the
use of M-x text-scale-mode RET.

Before this fix, M-x text-scale-mode RET was only available after
the agenda buffer was redone once.  Now it is available immediately.

This fixes commit 00c1184a0.

lisp/org-agenda.el

@@ -2199,7 +2199,6 @@ The following commands are available:
   (interactive)
   (let ((agenda-local-vars-to-keep
 	 '(text-scale-mode-amount
-	   text-scale-mode
 	   text-scale-mode-lighter
 	   face-remapping-alist))
 	(save (buffer-local-variables)))