Remove misleading comment

`caml_allc_custom_mem` cannot trigger finalization any longer.

runtime/io.c

@@ -505,7 +505,7 @@ static struct custom_operations channel_operations = {
 CAMLexport value caml_alloc_channel(struct channel *chan)
 {
   value res;
-  chan->refcount++;             /* prevent finalization during next alloc */
+  chan->refcount++;
   res = caml_alloc_custom_mem(&channel_operations, sizeof(struct channel *),
                               sizeof(struct channel));
   Channel(res) = chan;