Remove most IEntity usages from explosions (#5240)

* Remove most IEntity usages from Destructible and Explosions

* Perform a minute amount of cleanup

* Fix build

Content.Server/Destructible/Thresholds/DamageThreshold.cs

@@ -94,7 +94,7 @@ namespace Content.Server.Destructible.Thresholds
                 if (!entityManager.EntityExists(owner))
                     return;
 
-                behavior.Execute(owner, system, entityManager);
+                behavior.Execute(owner, system);
             }
         }
     }