I'm not sure I understand you well. You mean you've added one of the standard panels to the default sidebar, such as Addons, Advanced Configuration, etc. In that case, at the end of the sidebar there's a "+" that let's you uncheck anything you've added.
The changes are stored in your toolBars.json file:
"identifier": "SideBar",
[...]
"panels": [
"bookmarks",
"history",
"notes",
"passwords",
"transfers",
"addons"