vinyl: simplify vy_run_iterator_next_key

vy_run_iterator_next_key() has to special-case LE/LT for the first
and the last page. This is needed, because this function is used by
vy_read_iterator_seek() for starting iteration. Actually, there's no
point for vy_read_iterator_seek() to use vy_read_iterator_next_key() -
vy_read_iterator_next_pos() + vy_read_iterator_find_lsn() would be
enough. Bearing this in mind, simplify vy_run_iterator_next_key().