This removes the sast_configration_by_click feature flag, added in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/36432, and updates
the relevant documentation.

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/220573, which
itself is part of [Configuration UI for SAST][1].

[1]: https://gitlab.com/groups/gitlab-org/-/epics/3262