As a developer working with Wikidata, I want the entity data to not consume too much network traffic.
As someone hosting Wikibase instances, I want the entity data to not consume too much storage space.
**Problem:**
The JSON serialization of each statement contains a `"type"` field, which always has the value `"statement"`, wasting 19 bytes per Statement (before compression). The field used to be necessary back when Wikibase distinguished between Claims and Statements, but it has been hard-coded to always contain `"statement"` since 2015 (Wikibase DataModel Serialization 1.4.0).
**Example:**
```
$ curl -s https://www.wikidata.org/w/api.php -d action=wbgetentities -d ids=Q12345 -d props=claims -d format=json -d formatversion=2 | jq .entities.Q12345.claims.P1050[0]
{
"mainsnak": {
// ...
},
"type": "statement",
"id": "Q12345$1d677e6e-4bad-e489-bda8-7a0bd8ec38c0",
"rank": "normal",
"references": [
// ...
]
}
```
**Acceptance criteria:**
* API responses do not contain a `"type"` field for Statements (note: they must still contain a `"type"` field for data values!)
* Stored contents of new revisions do not contain a `"type"` field for Statements ([how to check](https://wikitech.wikimedia.org/wiki/User:Lucas_Werkmeister_(WMDE)/How_to_get_the_raw_text_of_a_page_or_revision))
* Non-goal: we do //not// update stored content of existing revisions.
NOTE: Like T217431, this is technically a breaking change that should be properly announced.