vinyl: get rid of obscure page cache promotion logic

vy_run_iterator_load_page() keeps two most recently read pages. This
makes sense, because we often probe a page for a better match. Keeping
two pages rather than just one makes sure we won't throw out the current
page if probing fails to find a better match. What doesn't make sense
though is cache promotion logic: we keep promoting the page containing
the current key. The comment says:

        /*
         * The cache is at least two pages. Ensure that
         * subsequent read keeps the cur_key in the cache
         * by moving its page to the start of LRU list.
         */
        vy_run_iterator_cache_touch(itr, cur_key_page_no);

The comment is quite misleading. The "cache" contains at most two pages.
Proudly calling this travesty of a cache LRU is downright ridiculous.

Anyway, touching the current page will simply swap the two cached pages
if a key history spans less than two pages, resulting in no performance
gain or loss whatsoever. However, if a key history spans more than two
pages, it will evict a page that is about to be read.

That said, let's get rid of this piece of crap.