Fix: invalid use of destructor in dynamic pointer array

A dynamic pointer array is built on top of a dynamic array and uses
the dynamic array's internal "destructor" field to store the
user-specified destructor.

lttng_dynamic_pointer_array_remove_pointer currently uses
the dynamic array's remove_element directly which causes the
user destructor to be called with the underlying storage of the
pointer rather than with the pointer itself.

This change re-uses the same pattern as
lttng_dynamic_pointer_array_reset(), namely using the destructor
explicitly and setting it to NULL for the duration of the call to
the dynamic array API.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>