Remove loggedin_user duplicate check for messages_check_thread_access()

[espellcaste]

On messages_check_thread_access(), the function is using a check for the logged in user that already happens on the BP_Messages_Thread::check_access.

I don't see why, so I'm suggesting to remove it.

[DJPaul]

Yes, this looks ok. Ping me or Boone on Slack sometime.

[espellcaste]

@DJPaul Any update about this?

[espellcaste]

In 11844:

Remove redundant handling of optional $user_id parameter from messages_check_thread_access()

On messages_check_thread_access(), we are checking if the $user_id parameter is set. The same check is being done on BP_Messages_Thread::check_access().

So we are keeping only the low level check, instead of both.

Props DjPaul.

Fixes #7668