The adaptive hash index is built on record prefixes on leaf pages,
containing at most index->n_uniq fields. Because the info_bits are
not part of the hash value calculation, the adaptive hash index
cannot distinguish the 'default row' record from a genuine user record
(and it cannot distinguish whether a record is delete-marked).

rec_fold(): Move the function to the only module where it is used,
and assert that the record is on a leaf page and not the 'default row'.

btr_search_drop_page_hash_index(), btr_search_build_page_hash_index():
Skip the 'default row'