[lib] Omit isDefault field in rawThreadInfoFromServerThreadInfo

Summary:
We're updating the specialRole client migrations to include exclusion of isDefault field. This is corresponding keyserver change to match that.

Depends on D11254

Test Plan:
Set the includeSpecialRoleFieldInRoles flag manually and observe that isDefault included before and excluded after.

Will attach screenshots.

Reviewers: tomek, ginsu, kamil, michal

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11261