[identity] helper functions for removing data from DDB

Summary:
on logout, we need to delete/remove the access token and keys associated with the logged out device.

the first helper function removes the device's keys from the users table with an update expression.

the second helper function deletes the device's access token from the access tokens table by calling delete_item().

Test Plan: created test data in local DDB and called the helper functions from main to confirm that the keys and token were removed

Reviewers: bartek, jon

Reviewed By: bartek, jon

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8500