Remove 1 -Wbad-function-cast warning in agdelsubg

Remove 1 -Wbad-function-cast warning in agdelsubg

Restores the public API to what's documented in cgraph.3 by instead of
casting the void pointer that dtdelete returns to long, returns an int
which is 0 if the pointer is NULL and 1 otherwise. Internally the
return value is however not used today.