Make HASH_DEL_IN_BKT look more like HASH_ADD_TO_BKT; refactor HASH_DELETE

[Quuxplusone]

HASH_DELETE now calls HASH_DELETE_HH, and avoids an ELMT_TO_HH conversion. Also, seeing that (head) in these cases (which really points to a bucket head, not a table head, so arguably it's confusingly named) can be a really complicated expression, let's pull it out into a local variable to help the optimizer.

No intended functional change.

(ping @veselov)

[veselov]

Absolutely