Removed unused argument 'strength' from LTXVImgToVideo.generate() method

[oumad]

When doing i2v the flow breaks at the LTXV Base Sampler node

This happens because the strength value gets passed to LTXVImgToVideo().generate which doesn't use it.

So in this PR I simply removed the strength in that method. And now the i2v flow runs correctly for me.