Revert: "fix: proxy-groups filter logic"

This reverts commit 8a85c63b08.

adapter/outboundgroup/loadbalance.go

@@ -28,10 +28,6 @@ type LoadBalance struct {
 	strategyFn strategyFn
 }
 
-func (lb *LoadBalance) IsProxyGroup() bool {
-	return true
-}
-
 var errStrategy = errors.New("unsupported strategy")
 
 func parseStrategy(config map[string]any) string {