removed MapConfig reference from MapProxySupport, now when it needs a Ma...

...pConfig it retrieves from the MapContainer, fixes #2810
There is no possibility to get null from getMapContainer(name) and getMapConfig() since they either return new instances or default values if the requested entries are not found. If the MapService or MapServiceContext is null, it will crash in the MapProxySupport constructor. So I cannot see any added value of pulling out local variables and null checking except styling.