Fix: Close traces on context destruction

bt_trace_handle_destroy is called on destruction of the trace_handle
hash table's elements. This function only frees the trace handle,
leaving the input traces open.

This fix sets remove_trace_handle as the value_destroy_func ensuring
that the format's close_trace function is called before the trace_handle
is destroyed.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>