What's this PR do?

This PR adds the ability to remove an external bank account from account.

Any background context you want to provide?

Right now, we are able to sync accounts (update/add) from current stripe data, but it would be great to have actions to do with deletions.

For example: User creates a custom account, which automatically creates an external bank account.

If this user changes their payment method and decides to add new account (using stripe API), we can use sync_account() to update our account/external_bank_accounts.

However, the user now wants to delete their previous (very first external bank account). We do this using stripe API:

account = stripe.Account.retrieve("acct_19c2PSCoyI6kORCB")
account.external_accounts.retrieve("ba_1CfV4JCoyI6kORCBWALOqwBd").delete()

This time, we need to manually remove this from BankAccount object, because syncing the data will not remove, but instead only add/update existing accounts.

What ticket or issue # does this fix?

Closes #569

Definition of Done (check if considered and/or addressed):

• Are all backwards incompatible changes documented in this PR?
• Have all new dependencies been documented in this PR?
• [] Has the appropriate documentation been updated (if applicable)?
• [] Have you written tests to prove this change works (if applicable)?