pcsx2: purge fmv in sw hack

[tadanokojin]

Rationale:
The hack was recently broken on master. We could fix it, but it's sort of pointless. Additionally, #3549 will break it as it forces the window to close when the renderer switches. This is desired as sharing a surface between different renderers is bad behavior.

At it's best, it offered a users a way to work around that some FMVs are not properly displayed by the HW renderer due to limitations of the cache by automatically switching to the more compatible sw renderer when they played.

At it's worst, it's a threading nightmare that barely worked and was never going to be a long term solution especially seeing as it conflicts with future changes that we would like to make.

Instead, I'll purge it and we'll work on making the HW renderer better instead of relying on a hack. Games which require sw mode will be tracked here: #3805

close: #3804