NetEvWait: Don't delete event in destructor

`NetEvWait` deletes the event that is assigned to it when itself
is deleted. But the event is not owned by the `NetEvWait`, it is shared among
all consumers of the event. Deleting it when the `NetEvWait` is deleted can
result in undefined behavior.

This is mainly a problem for non-blocking event control assignments with a
zero or negative immediate valued repeat. In this case the `NetEvWait` will
be deleted as it is not needed.

```
reg x;
event e;
x <= repeat(1) @e 1'b0;
x <= repeat(0) @e 1'b1; // Assert triggered since in-use event is freed
```

Remove the delete to fix this. Events that end up being unused will be
freed by the nodangle functor.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

net_event.cc

@@ -407,7 +407,6 @@ NetEvWait::~NetEvWait()
 			tmp->next = tmp->next->next;
 			delete tmp;
 		  }
-		  delete tgt;
 	    }
 	    events_.clear();
       }