Fix trace UST destroy channel

The trace_ust_destroy_channel call was iterating over the internal
events hash table and destroying the complete hash table at each event
which makes no sense at all. This caused a major segfault in the hash
table data structure when destroying a session.

Reported-by: Tan Dung Le Tran<tan.dung.le.tran@ericsson.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>