[keyserver] Remove table prefix in sql query

Summary: Fix for this comment: https://phab.comm.dev/D9598#inline-60562. We don't need to use the full table name.

Test Plan: Create a new user, check if their membership.last_message for genesis was set to the latest text message.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9629