Various fixes upport from chomp (#15794)

* up-port

* more upstream commonality

code/ATMOSPHERICS/datum_pipe_network.dm

@@ -1,5 +1,3 @@
-var/global/list/datum/pipe_network/pipe_networks = list()	// TODO - Move into SSmachines
-
 /datum/pipe_network
 	var/list/datum/gas_mixture/gases = list() //All of the gas_mixtures continuously connected in this network
 	var/volume = 0	//caches the total volume for atmos machines to use in gas calculations

code/ATMOSPHERICS/pipes/simple.dm

@@ -96,7 +96,7 @@
 		node1 = null
 	if(node2)
 		node2.disconnect(src)
-		node1 = null
+		node2 = null
 
 	. = ..()