To remove, you need to focus on one side of the dowel perch. Slide it along the bar of the cage that it's attached to, in this case horizontally. Do not slide the whole perch, just one side of it. This will loosen it and you can remove it. If for whatever reason this isn't possible then I'm certain you can use a little elbow grease and force it out. I guess we're all just a bit confused what the problem is and why sliding it out isn't working.
Regarding your cage setup, I would be WAY more concerned with ADDING things to that cage at this point than removing them. An inappropriately sized dowel perch can cause bumble foot if it's the only option. (A dowel perch that is the right size, ie that is large enough or even maybe a bit "too" large, is not inherently harmful.)
To avoid problems with sores on the feet you need plenty of different surfaces for your bird to perch on. Right now you don't have nearly enough. Also, someone else posted in another thread regarding a cage setup for their quaker and that their child was trying to remove a dowel perch, is that you? They mentioned having the type of perch that files nails--if you have that in your cage then it's way more important to take that out than the dowel. Those are dangerous.