[keyserver] Remove holders in account deleters

Summary:
Address ENG-9353.
Before calling DELETE FROM uploads, we need to select the extra column first and remove blob holders if present in the column JSON.

Depends on D13513

Test Plan: Enabled blob-hosted user avatars. Created an account, set image avatar, then deleted the account and watched the logs. Similiarly to D13513, a blob was uploaded and then its holder was removed

Reviewers: tomek, ashoat

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D13514