Fix : double destroy in context remove

The hashtable storing the trace handle is created with a destroy
function that frees the elements when they are removed. Manually calling
the destroy function after removing a handle for the table causes
double-free problems.

Fixes #197

Reported-by: Mathieu Denis <mathieu.denis@polymtl.ca>
Signed-off-by: Julien Desfossez <julien.desfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>