Triplelift Bid Adapter: stop override on placement

[patmmccann]

fixes #10407

[nllerandi3lift]

Is this necessary even with placement:1 being the only true instream value? Is not context:'instream', placement:2 (or any other value) just wrong? Otherwise I see no harm here, but thought I'd fire this Q off first.

[patmmccann]

Some publishers have been instructed by DSPs that they must set video.placement equal to a value other than one when they have muted pre-roll. In this case video.context will remain 'instream' even though video.placement has some other value.