[lib] Allow calling createRealThreadFromPendingThread for PRIVATE thread

Summary:
Not sure why this invariant is here (added in D13244), but I tested and it all works correctly after removing it.

Depends on D13547

Test Plan: On both native and web, tested PRIVATE thread creation end-to-end via thread list search, chat composer, and user profile. Confirmed that a peer (same user device) receives the DM messages

Reviewers: tomek, kamil, inka

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D13548