I'm thinking of removing a brake(front or back) to reduce weight and simplify my bike. Which brake is most expendable? Its your average fixie road bike.
This is an extremely bad idea, and it may or may not be illegal where you live. It's pretty common for the law to require you to be able to lock up both wheels on flat dry pavement.
Regardless, both brakes are important, and you shouldn't remove one for something a silly as weight reduction. The contact patch on your tires is small enough already without throwing away half your stopping power.
Your rear brake will slow you without impacting your steering, while your front break is responsible for ~60% of your stopping power. However, braking with your front brake while turning sharply will usually result in loss of traction of your front wheel, causing the bike to slide out from under you, so it's crucial that you keep your rear brake.
If you're still dead set on this course, remove the rear brake, it's the less effective of the two. Some day when you're cornering and a car pulls out in front of you, and your options are to brake and slide out or hit the car, you'll regret not having both brakes.
Keep your front brake. It does the most work, it will stop you much faster than your rear brake ever could hope to. Take a look at a motorcycle, the front brakes are always much larger than the rear. Whenever you brake, on a bicycle, motorcycle, in a car, more weight is transfered to the front wheels, so the front tire has more traction to stop you with.
That said, sit down a minute and think about what you're going to do if you remove your rear brake, and one day your front brake stops working while you're going down a hill. Right about then you'll wish you had just put up with the extra 400 grams of weight.