Remove record_migrations=True from tutorials. #290

As @jeromekelleher says:

to be clear, I regard the record_migrations option in msprime to be a legacy feature that has been superseded by the additional nodes feature, which provides the same information as nodes explicitly marking migrations and edges, and is much simpler.

record_migrations only occurs in 2 places. The only tricky one to remove is in https://github.com/tskit-dev/tutorials/blob/main/introgression.md.