Revert "Make sure teachers can't upload videos into arbitrary series in single-upload"

[ferishili]

Reverts #363

[NinaHerrmann]

Why do you think this should be reverted? I won't be available for a meeting today, we can appoint one tomorrow if you think it is not explainable in text.

[ferishili]

For the reason I mentioned in that belated review:
The fix has no use to the security issue, because if you pass arbitrary series:

• by query param: it will never reach to the point that you can select it or put it in the form to submit.
• If one can submit anything to the addvideo i.e. arbitrary series id via POST maliciously: then the provided fix to give null value to $series does NOT have any effect!

I can explain it in more details in a live meeting and go through the process step by step