fix label of --id in admin delete user (#14005)

fix label of --id in admin delete user (#14005)

* fix label of --id in admin delete user

This pr fixes the label descriptor of `gitea admin delete user`
but also adds a `--username` option.

Fix #13995

Signed-off-by: Andrew Thornton <art27@cantab.net>
* fix-spacing

Signed-off-by: Andrew Thornton <art27@cantab.net>
* Add delete email support

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>