refactor(BA-5712): clean up stale slice C fragment and comment

- Drop the leftover ``changes/BA-5650-C.misc.md`` file that was
  superseded when slice C's fragment was renamed to
  ``changes/11045.enhance.md``.
- Remove the explanatory comment about ``user_uuid`` being non-nullable
  from the ``update_session_name`` path — it restates what the
  surrounding code already makes clear.

Author: jopemachine

changes/BA-5650-C.misc.md

@@ -306,9 +306,6 @@ async def modify_session(
                 raise SessionNotFound(f"Session not found (id:{session_id})")
 
             if session_name:
-                # ``SessionRow.user_uuid`` is non-nullable, so we only need to
-                # gate on whether a new name was provided.
-                # Check the owner of the target session has any session with the same name
                 try:
                     sess = await SessionRow.get_session(
                         db_session,