git: Replace git-signoff with rebase --signoff

Replace my custom git-signoff script with the native `--signoff` flag
for `git rebase`.

.config/zsh/autoload/git/git-signoff

@@ -1,21 +0,0 @@
-#!/usr/bin/env zsh
-## Author:  Julian Prein <druckdev@protonmail.com>
-## Created: 2022-03-29
-##
-## Signoff range of commits
-
-# Exit if not in git repo
-git rev-parse || return
-
-# Exit if no arguments were given
-[[ $# -gt 0 ]] || return
-
-# Signoff all commits
-env GIT_SEQUENCE_EDITOR="sed -Ei 's/^p(ick)?/e/'" git rebase -i "$@" || return
-
-while git rebase --show-current-patch &>/dev/null; do
-	git commit --amend --signoff --no-edit --no-verify
-	git rebase --continue
-done
-
-return 0