Replace JSON schema with the one in nimbus-schemas

[mikewilli]

mozilla-nimbus-schemas is now publishing JSON schemas with the python package, so we should be able to use that instead of keeping our own copy in the repo for testing.

Should be something like

import importlib.resources

schema_path = importlib.resources.files('mozilla_nimbus_schemas') / "schemas" / "NimbusExperiment.schema.json"

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