Remove todo notes

They have either been implemented or filed as an issue on GitLab.

todo.txt

-Basics practical:
-- Make a ``projects directory''.
-- Make an empty directory named ``my_project'' and do a ``git init'' there.
-
-Annex practical:
-- Merge the two sub-practicals into one coherent practical.
-- Check the ``git annex drop'' part, it needs a sync first.
-
-General:
-- Build on one example repository, to make everything more coherent.
-- End all presentations with one slide that summarizes all important commands
-  (show it during practical).
-- If we have time, create (wiki) pages with the same content as the slides
-  (they can be more verbose), also useful outside the course.
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