how do you remove the pin?
It is illegal to do in California or any other state where you can't own a magazine over 10 rounds.
I don't care to speculate how any loopholes or build kits may work around it. It's not worth the legal trouble when it's $20 for another magazine.
If you can do so legally all you need to do is drill the bit of plastic on either side to expose the ends of the pin and tap it out. This will obviously permanently alter the magazine. There is a thread with pictures if you look.