Remove a setter for a frozen dataclass

You cannot set that anyway.

birdvisu/topo_v3.py

@@ -191,10 +191,6 @@ class VertexID:
 
 	@property
 	def is_network(self): return not self.is_router
-	@is_network.setter
-	def _set_network(self, val: bool):
-		assert isinstance(val, bool)
-		self.is_router = not val
 
 @dataclass
 class Vertex: