plugin: fix static check issues (pingcap#13881)

Author: Rustin170506

plugin/conn_ip_example/conn_ip_example.go

@@ -45,5 +45,4 @@ func OnGeneralEvent(ctx context.Context, sctx *variable.SessionVars, event plugi
 	fmt.Println("conn_ip_example notifiy called")
 	fmt.Println("variable test: ", variable.GetSysVar("conn_ip_example_test_variable").Value)
 	fmt.Printf("new connection by %s\n", ctx.Value("ip"))
-	return
 }

plugin/plugin.go

@@ -59,9 +59,7 @@ func (p *plugins) clone() *plugins {
 	for key, value := range p.versions {
 		np.versions[key] = value
 	}
-	for key, value := range p.dyingPlugins {
-		np.dyingPlugins[key] = value
-	}
+	copy(np.dyingPlugins, p.dyingPlugins)
 	return np
 }