bt2: remove some unnecessary checks

bt2: remove some unnecessary checks

 - getting the id from a stream class, stream or event class can't
   return negative values
 - getting the trace class from a trace can't return None
 - getting the stream class from a stream can't return None

Change-Id: If797994f4a10072f662c122c15a5081419d9fa3d
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13502
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>