I have deleted all images and all instances, yet I cannot remove network: lxdbr0. I have shutdown lxd as well. The story is the same with storage as well. Why? I have lxd clustering enabled.
I completely forgot about projects. I switched to the other projects and removed the unused images and done it is fixed.
Haha, that’d do it 
Worth noting that lxc network show and lxc storage show in this case should have been told you what’s using those resources (used_by field).
I know, but I couldn’t find who was using it. But thanks, your comments are my very important source of finding solution to my problems.