We've made a pull request and now we can clean up the branches by deleting the feature branch.
Branches are just pointers to commits - so we can safely delete branches without losing the underlying commits (once the commits are merged back into master).
So we'll use the github interface to delete the branch remotely, and to delete it locally we'll use
git remote prune origin --dry-run
and then
git remote prune origin
That will tell us that the remote is gone, and we can finally clean up the feature branch with:
git branch -D feature-branch
Also we can always using
git reflog
To restore any deleted branch