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 [[ https://linear.app/comm/issue/ENG-5833/update-code-that-uses-default-role-from-mariadb | ENG-5833 ]]