I put this call in by habit, rather than because there was any
actual need. The filter is simply processing frames one after
the other and has no need to synchronise.

malakudi on the devtalk forums noticed a slowdown when using nvenc
with temporal/spatial aq and that the slowdown went away if the
sync call was removed. I also verified that in the basic encoding
case there's an observable speedup.

I also verified that we aren't doing unnecessary sync calls in any
other filter.