[identity] Delete Backup user data on account deletion

Summary:
Similarly to how we call Tunnelbroker, called Backup Service DELETE /user_data endpoint from D12970 to delete backup data
in DeleteUser RPCs.

Depends on D12972

Test Plan: Called the DeleteWalletUser RPC. Verified that Backup is called and user data is actually deleted from Backup DDB

Reviewers: will, varun, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12973