delete user

WP [#17654](https://community.openproject.org/work_packages/17654)

Requires merging of #2325
@machisuji I would have expected something similar to https://github.com/opf/openproject/pull/2331/files#diff-c423206e7cb9a4649ce7e0ea5512093eR48 here in this controller. Although I have to admit that it hasn't been there before as well.