Get rid of api.py temporary directory for sample storage logic

[jbremer]

When submitting a task through api.py a temporary directory is created with the submitted file in it. This temporary is used to preserve the filename of the sample. However, as this information is also available in the database, it's not actually required, and thus the logic of the temporary directory can be removed.
The actual bug is the fact that the temporary directory is not actually removed upon deleting the task, thus leaving thousands upon thousands temporary directories around (assuming you have that many tasks).

[botherder]

It is currently used by both api.py as well as the Django interface. Checking if we can safely get rid of it.