Create a mutation to remove a secondary manager from a Billing Account

Resources

1. Design: Figma
2. Implementation plan: Blueprint
3. Main epic: Enable customers to self-service manage multipl... (&10495)

Proposal

API Contract defined in https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/main/doc/architecture/multiple_billing_account_managers.md#api-contract

We need to be able to delete memberships for Customers that should no longer be able to manage a billing account. Create a mutation that facilitates this.

Requirements:

1. The target Customer cannot be the bill_to or sold_to on the account