Remove old code mitigating jsonschema Draftv4

A bunch of code:
- Here: https://github.com/Kinto/kinto/blob/ea5f66dd4441447292ce26a068bdb690b7de7496/kinto/schema_validation.py#L38-L44
- And here: https://github.com/Kinto/kinto/blob/ea5f66dd4441447292ce26a068bdb690b7de7496/kinto/schema_validation.py#L59-L65

Don't necessarily have sense anymore since we moved to Draftv7.

I believe this should be a follow up of the release of jsonschema 3.0 as a first stable version and would block #1916

Start in kinto/schema_validation.py at the two cited ranges and review how the code relates to the move from JSON Schema Draftv4 to Draftv7. Remove the obsolete mitigation once the jsonschema 3.0 follow-up context is confirmed, and verify that schema validation still behaves as expected before addressing the dependency on #1916.

Written by the indexing model from the issue text.