Introduce pipeline schedules refactor

What does this MR do and why?

Last MR in the huge effort to rebuild Pipeline Schedules feature in Vue/GraphQL.

View the epic here &6189 (closed)

1. Removes the pipeline_schedules_vue feature flag and the old code.
2. Updates test coverage to not stub the flag

Notes to reviewers

This is a large removal, I.E lots of diff removal changes. Best way to review this MR is to ensure everything is cleaned up well (pipeline_schedules_vue) and that the feature is working as intended Build > Pipeline schedules. And for the spec reviewer to ensure we didn't lose any coverage with the feature spec. It looks scary with the diff removal, but it in fact should be a pretty easy MR to review.

Screenshots or screen recordings

How to set up and validate locally

1. Visit Project > Build > Pipeline schedules
2. Verify everything is working

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 #375139 (closed)