ggml : remove unnecessary UNUSED macro call (ggml/880)

This commit removes an UNUSED macro call that is not needed as the
variable n0 is used in the code and will not produce a warning.

Signed-off-by: Daniel Bevenius <daniel.bevenius@gmail.com>

ggml/src/ggml.c

@@ -18078,7 +18078,6 @@ static void ggml_build_forward_impl(struct ggml_cgraph * cgraph, struct ggml_ten
     }
 
     const int n0 = cgraph->n_nodes;
-    UNUSED(n0);
 
     ggml_visit_parents(cgraph, tensor);