perf(elastic): filter system fields in es instead of in whelk

[olovy]

Less data that is just transferred and discarded for each query.

catches everything:

$ curl -s "http://localhost:8180/find?_q=" | gron | grep items | cut -d'=' -f1 | grep _ | grep -v _categoryByCollection
$

fixes ..._4_digits_... that slipped through before:

curl -s "https://libris-qa.kb.se/find?_q=" | gron | grep items | cut -d'=' -f1 | grep _ | grep -v _categoryByCollection
json.items[0]["@reverse"].instanceOf[0].publication[0].year_4_digits_keyword
json.items[0]["@reverse"].instanceOf[0].publication[0].year_4_digits_short
json.items[0]["@reverse"].instanceOf[0].publication[0]["librissearch:year_4_digits_keyword"]
json.items[0]["@reverse"].instanceOf[0].publication[0]["librissearch:year_4_digits_short"]
...

Couldn't see any slowdown in response times from elastic.

[kwahlin]

LGTM!

[olovy]

LGTM!

updated
refactor(elastic): constants for SystemFields
chore(elastic): remove dead code