How can I remove brush marks from a wall painted with metallic paint?
Once dry, brush marks will not go away without sanding...sorry.

Once that is done though:

When you paint any given space you should move precisely and quickly.  So in this case when you cut out the space in your kitchen (brushing the edges) you should immediately (before the cut work dries) go over it with the roller and get the roller as close to the edge as is reasonable.  

Even with good brushes (I agree with the poster who mentions using quality applicators, I was a painter for 10 years or so and they make a ton of difference) you will still see a difference between the sheen on the portions that are brushed and the portions that are rolled but this can be minimized if you roll over the cut portions before they dry as it makes the sheen/texture match.  

Edit:  Also, if you are using the same paint again there is absolutely no reason to prime after you sand down the brush marks.