Edit the Places panel in Dolphin
The Places panel in Dolphin fills up fast. Every removable drive you've ever plugged in, every network share, every folder some application decided you'd want a shortcut to.
Cleaning it up is easy once you know where to look.
Press F9 to open the panel if it isn't visible, then right-click any entry and choose Remove. You can also Hide an entry if you'd rather keep it around but out of sight, and drag entries to reorder them.
If you'd rather do it in bulk, or you're scripting a new machine, the whole thing lives in a single file:
vim ~/.local/share/user-places.xbel
It's plain XML, one <bookmark> block per entry. Close Dolphin before editing it, otherwise it'll overwrite your changes when it exits.
Small thing, but a tidy sidebar is worth the two minutes.