repoDir = "pathtoexistingrepo"
r, err := git.PlainOpen(repoDir)
if err != nil {
	return err
}

branchToDelete = "test"
branches, err := r.Branches()
err = branches.ForEach(func(b *plumbing.Reference) error {
	if b.Name().String() == branchToDelete {
		fmt.Println("Found the branch")
		err = r.DeleteBranch(b.Name().String())
		if err != nil {
			return err
		}
	}
	return nil
})
if err != nil {
    fmt.Println(err)
}