[lib] Remove isDefault field from RoleInfo

Summary:
We previously removed all usages of RoleInfo.isDefault on the native/web client, so we can safely remove it from RoleInfo.

As usual, there will be a native/web migration to go along with this change. Will actually update the existing specialRole migration in place since it makes sense to couple the inclusion of specialRole and exclusion of isDefault in a single pass.

Depends on D11252

Test Plan:
flow + verified that RoleInfos sent from the keyserver have the isDefault field excluded.

Did a search through the codebase to ensure there were not any instances of isDefault that are relevant to changing this client type.

Reviewers: ginsu, tomek, michal, kamil

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11253