delete user

WP [#17654](https://community.openproject.org/work_packages/17654)

Requires merging of #2325
@machisuji now I see it, https://github.com/opf/openproject/pull/2331/files#diff-4e05ad0d64e6100656b63ad1e78f32c5R333. Sorry for being hasty. But wouldn't it have been better if that before filter was removed in favor of an if/else block?