Remove data embedding from chart in reef-check.ipynb

We're using quiltdata to render charts and our current chart ([last cell](https://open.quiltdata.com/b/ai-on-the-beach/tree/aionthebeach/reef-check/reef-check.ipynb)) is > 6MB so we want to stop embedding data.

> Vega and vega-lite don’t require data embedding (the chart can reference data in a CSV or other external source). What I’m hoping to find is a way to use Python to create vega specs w/o embedding the source data.

-- [Kevin @quilt](https://github.com/kevinemoore)