Remove custom_roles_ui-[xxx] feature flags

What does this MR do and why?

Rollout issues:

• [Feature flag] Enable custom roles and permissi... (#423077 - closed)
• [Feature flag] Enable custom roles and permissi... (#416644 - closed)

custom_roles_ui_self_managed and custom_roles_ui_self_managed have been enabled by default since 16.4, !131930 (merged).

EE: true

Changelog: other

This MR doesn't need Changelog: changed because it was added when the feature flags were enabled by default (see link above).

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.