Investiage removal of is_active in vandar_mandates

[MikeElahi]

Since we have different status fields in the process of creating, activating, and deleting mandates, along with an "active" scope that runs a query on the database, the use of "is_active" in Cashier seems unnecessary.

• Investigate the usage of the is_active field
• Make sure there's a solid scenario for deletion of Mandates