PostgreSQL: Remove feature toggle postgresDSUsePGX (#113675)

* PostgreSQL: Remove feature toggle `postgresDSUsePGX`

* Fix tests and linting

* Address review comments

pkg/services/pluginsintegration/plugintest/plugins_test.go

@@ -161,7 +161,7 @@ func TestIntegrationPluginManager(t *testing.T) {
 	pr := prometheus.ProvideService(hcp)
 	tmpo := tempo.ProvideService(hcp, tracer)
 	td := testdatasource.ProvideService()
-	pg := postgres.ProvideService(cfg)
+	pg := postgres.ProvideService()
 	my := mysql.ProvideService()
 	ms := mssql.ProvideService(cfg)
 	db := db.InitTestDB(t, sqlstore.InitTestDBOpt{Cfg: cfg})