Manual: remove the old todo file [skip CI]
Manual: remove the old todo file [skip CI]

The preferred way to track required updates to the manual is now
to use Github issues.
eg: https://github.com/gem/oq-engine/issues/3018

Former-commit-id: b103afa58daf6f8c4d99b62ed6aa4c34eeba33b8 [formerly 2545d7a9dfbf6256c640c41b358bd4afbffc89df]
Former-commit-id: 16edfd96465807fbf62a61d592974e68b3a6d6f6
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