Remove duplicated tasks.

Some tasks are seen duplicated of another task or tasks.  They are
wrappers and dequeuer of others.  They would be looked as an outer
level of nested tasks.  It is quite annonying and useless for the
users of TaskTracer, so they are removed at the converter.
Merge pull request #6 from ThinkerYzu/remove-dup-tasks

Remove duplicated tasks.