How do you Un-Register Custom WP Sidebar Plugins?

Hey there,
We did a site migration, rebuilt the site, and now we have some left-over elements we would like to delete. Unfortunately, these elements aren’t listed in our plugins, which means they must be registered elsewhere.

Can someone tell me where I might find/delete things from the dashboard sidebar that are showing up where a new plugin would when active, when it’s not in the plugins folder?

Thank you!

Wants to remove:

Those might be custom post types. Check out the code in your functions.php file.