Finalize diff fields migration

Commit 862b6a3ea2 and 5e89f43747 moved all /diff_.+/ fields from the
snapshot model to the new snapshot_diff model. Before I removed the old
fields, I wanted to make sure people had enough time to transfer data
(this was all communicated in isssue #65 [1]).

A week has passed since then, so I'm moving ahead on deleting these
fields.

[1] https://github.com/trotzig/diffux/issues/65