Cleaning up after yourself is important, and not just in real life. Good Git Hygiene™ goes a long way.
One of the methods I like to clean up is deleting unused feature branches. I do that both locally and on the remote source control server (github/gitlab etc).
As is common with Git there are many ways to feed a cat. Some people use this:
git push origin :name-of-branch
I prefer the more declerative way, especially for potentially destructive operations such as deleting a remote branch:
git push origin --delete name-of-branch
Either way, keeping your remote branches trim makes for a happier development team!