[keyserver] Update usage of threads.default_role in thread-creator.js

Summary:
We need to get rid of usages of threads.default_role throughout the codebase so we can eventually drop the column.

This diff handles the usage in thread-creator.js

Existing code: When creating a thread, the roles from createInitialRolesForNewThread are used to populate the role ID into threads.default_role for that specific row. We should no longer populate this field.

Depends on D9980

Addresses part of ENG-5833

Test Plan:

1. Ran ALTER TABLE threads MODIFY COLUMN default_role bigint(20) NULL; to allow NULL values in the column for now (until the column is dropped)
2. Created a new thread and checked to make sure that no role got inserted into default_role

Reviewers: atul, ginsu, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9981