Add ability to delete pipeline subscriptions

What does this MR do and why?

Part three: Refactoring pipeline subscriptions feature to Vue/Graphql

Goal of this MR

1. Add ability to delete pipeline subscription
2. Build confirmation modal
3. Add test coverage

You can view rough outline of the following steps during this refactor here &11446

Part one: !139405 (merged)

Part two: !140812 (merged)

Part three: You are here...

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable feature flag in rails console pipeline_subscriptions_vue
2. Visit Project > Settings > CI/CD > Expand pipeline subscriptions
3. Verify everything is working as expected

Related to #425294 (closed)