there should be a screw on the back side (from the view in your picture) that you can unscrew to loosen the gooseneck's grip on the lower boom. it might be difficult with yours, as there seems to be a bit of corrosion(?), so it might take some work. it might help to get some bolt thread loosener, too.
Now that you have the bolt out (as shown in your photo) take a large flat head screwdriver and insert it in the gap where the bolt was. Twist to open the gap and the gooseneck should slide right off. If not, gently hammer the screwdriver in to open the gap more and gently tap the side if the fitting with a small hammer to move it down the boom. You will then have to remove the end cap eye bolt to get the gooseneck completely off the boom. The gooseneck should them slide off the over the plastic end cap without having to remove the cap.