I have a Jack Madden and wondered the same thing so I took it out and had a look to see if I could lose any weight. It wasn't a weight bolt but just a short aluminum bolt that was threaded down the inside. I didn't think much of it so I left it out and threw it in a drawer.
A year later the butt plate fell off cracking the finish.
I would recommend leaving it in. It's structural.
JC