nak: Remove assert on nir->info.outputs_written
nak: Remove assert on nir->info.outputs_written
nir->info.outputs_written isn't used for fragment shaders except as an
early out a few lines above this, so we don't rely on this property.
My best guess is that this was intended to check if the information
from nir_gather_info is stale, but dead variables fail the assert
even if the info is up to date.