Allow on call schedule deletion

What does this MR do?

A tiny MVC for #262850 (closed). This merge allows for the deletion of oncall schedules through a modal pop-up, and is based on backend work that is yet to be merged.

Design

https://gitlab.com/gitlab-org/gitlab/uploads/afa2f7e5fcb692fb522887ca461ebc33/Delete_confirmation.png

Screenshots (strongly suggested)

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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Related to #262850 (closed)