Code Insights: Remove insights api instance checks

Created by: vovakulikov

Closes part of https://github.com/sourcegraph/sourcegraph/issues/31412

Background

Historically we've been dealing with two different types of API in our UI. Setting-based API (currently deprecated) and GQL real API. We unified most of the methods of APIs when we were implementing the GQL API adoption but we left a few places in our UI component where we didn't extract the Setting-based API in a generic API class. This PR removes these places.

Test plan

• Check that the creation UI navigation works properly (dashboardId URL query param should still redirect users to the right dashboard after creation)
• Check that insight creation/editing works properly for all types of insights