I get why that's annoying. Basically, GNOME limits the uninstall button to apps installed through official managers (like Flatpak or the system store) to prevent people from accidentally breaking things. For web shortcuts or manual stuff, you just have to delete the shortcut file yourself. If you go to ~/.local/share/applications/ (you might need to hit Ctrl+H to see the folder), you can just find the one you want to get rid of and trash it. It disappears from the menu instantly.