org.el: Improve accuracy of message

* lisp/org.el (org-remove-file): Reword success message.

Before this change, when `org-remove-file' succeeded in removing a
file from `org-agenda-files', it would display this unduly alarming
message to the user: "Removed file: foo.org".  This made it seem as
though the file itself were removed from the filesystem, rather than
simply being delisted from `org-agenda-files'.

After this change, the message "Removed from Org Agenda list: foo.org"
is displayed instead, so the user will experience only the normal
level of panic involved in using Org Mode, not the increased panic
that results from thinking a file has been removed when it hasn't.

TINYCHANGE

lisp/org.el

@@ -18091,7 +18091,7 @@ Optional argument FILE means use this file instead of the current."
 	(progn
 	  (org-store-new-agenda-file-list files)
 	  (org-install-agenda-files-menu)
-	  (message "Removed file: %s" afile))
+	  (message "Removed from Org Agenda list: %s" afile))
       (message "File was not in list: %s (not removed)" afile))))
 
 (defun org-file-menu-entry (file)