NCRoomsManager is retained when opening threads from conversation list

## Steps to reproduce
1. In conversation list tab "Threads"
2. Open a thread
3. Close it via "Close" button

-> NCRoomsManager is retained, as the NCRoomController is not removed. The issue comes from the return statement introduced at https://github.com/nextcloud/talk-ios/commit/d00bf314b5fe34992b7798a84fcfe5cd6ee6aa56. I _think_ we can only return here if it's a threadview controller that is shown from a chat view, but not without it?

main @ 3abb1777e9fbb8c32558fbc925324e7317fb06ec

Reproduce the flow from the conversation list's Threads tab by opening and closing a thread, then inspect NCRoomsManager and NCRoomController around the return statement introduced in commit d00bf314b5fe34992b7798a84fcfe5cd6ee6aa56. Trace whether the controller is removed when the thread is opened from a conversation list rather than a chat view. Done means closing the thread no longer retains NCRoomsManager.

Written by the indexing model from the issue text.