bug#5545: iswitchb assumes bookmark-alist is bound

[Chong Yidong]

> In the current trunk, since 2010-02-03:
>
> Compiler warning:
>
> iswitchb.el:1293:19:Warning: reference to free variable `bookmark-alist'
>
> Example:
>
> emacs -Q \
>    --eval '(progn (setq iswitchb-use-virtual-buffers t) (iswitchb-mode))'
>
> C-x b 1
>
> -> Error in post-command-hook: (void-variable bookmark-alist)
>
> The documentation of iswitchb-use-virtual-buffers does not mention
> searching bookmarks, it only mentions recentf. If it is non-nil, it
> activates recentf-mode, but there is no such feature for bookmarks.
>
> Also there is no NEWS entry for this change.

I don't think this change is consistent with the feature freeze, so I
have reverted it for now.  We can revisit this after the release.