Don't use API to fetch alternate poster sizes

Kind of weird to fetch it so indirectly.
Also appearently causes problems with certain maintenance
scripts when running on private wikis.

Note: TimedText still accesses the API internally. I do not
know if that would affect maintenance scripts.

Bug: 54394
Change-Id: I32b933035d76d44854c2839d727061ae41c5dc30
Merge "Don't use API to fetch alternate poster sizes"