Remove S3 paths from notebooks

[willettk]

As pointed out by @harsh-parikh, there are several S3 buckets mentioned in the new notebook. S3 buckets shouldn't be included in any commits to this repo (even if access is restricted) unless the bucket is fully public and has been approved. Will have to keep those references in internal documentation.

https://github.com/amzn/credence-to-causal-estimation/blob/credence_new_structure_new_marginal_likelihood/notebooks/example-new-structure.ipynb

[willettk]

One possibility would be adding a pre-commit hook to check for S3 paths before allowing commits? Maybe something like git-secrets.

[Louisexu925]

S3 buckets are removed from all branches with the same notebook. And all branches pass git-secrets scans, after adding a specific pattern for s3 paths.