Remove analytics dashboards editor feature flag

What does this MR do and why?

This MR cleans up the combined_analytics_dashboards_editor feature flag for [Feature flag] Rollout of `combined_analytics_d... (#411407 - closed). Why? The feature flag has been enabled and can now be removed. This is a sub-feature of analytics dashboards and thus this is still hidden behind the product_analytics_dashboards and combined_analytics_dashboards feature flags.

Screenshots or screen recordings

Dashboards list

Before (ff disabled)	Before (ff enabled)	After

Dashboard page

Before (ff disabled)	Before (ff enabled)	After

Builtin dashboard page

No visual change.

Before (ff disabled)	Before (ff enabled)	After

Custom dashboards not configured

No visual change.

Before (ff disabled)	Before (ff enabled)	After

How to set up and validate locally

1. Follow these instructions to setup Product Analytics in GDK.
2. View the dashboards list at Project > Analytics > Analytics dashboards.
3. Set up Product Analytics and follow the steps to complete onboarding.
4. View the dashboards list at Project > Analytics > Analytics dashboards again and validate that it lists:
   • Audience
   • Behaviour
   • Value Stream Dashboard
   • And any other custom dashboard created

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.

Related to #411407 (closed)