[identity] delete all token data on user account deletion

Summary:
new db method to get all tokens for a user and then delete them with BatchWriteItem. we use the comm-lib batch_write function for the delete requests.

Depends on D11795

Test Plan: signed in to a local account from web, native, and keyserver. then deleted my account from native. confirmed that there were no tokens in local ddb for my user ID

Reviewers: bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11796