bt2: make it possible to remove a trace class or trace destruction listener

bt2: make it possible to remove a trace class or trace destruction listener

This patch adds the possibility to remove a previously added trace or
trace class destruction listener.  Calling
{_TraceClass,_Trace}.add_destruction_listener returns a
_ListenerHandle.  To remove the listener, the user must pass this handle
back to {_TraceClass,_Trace}.remove_destruction_listener.

Change-Id: I6020135c1e056658d941d2e022b171466c5df7b3
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2077
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>