Prometheus: Remove buffered client and feature toggle related to it (#59898)

* Remove prometheus buffered client and feature toggle related to it

* Remove redundant pieces

* Clean the integration test

pkg/services/featuremgmt/registry.go

@@ -214,11 +214,6 @@ var (
 			State:        FeatureStateAlpha,
 			FrontendOnly: true,
 		},
-		{
-			Name:        "prometheusBufferedClient",
-			Description: "Enable buffered (old) client for Prometheus datasource as default instead of streaming JSON parser client (new)",
-			State:       FeatureStateStable,
-		},
 		{
 			Name:        "newDBLibrary",
 			Description: "Use jmoiron/sqlx rather than xorm for a few backend services",

pkg/services/featuremgmt/toggles_gen.go

@@ -155,10 +155,6 @@ const (
 	// Enable trace to metrics links
 	FlagTraceToMetrics = "traceToMetrics"
 
-	// FlagPrometheusBufferedClient
-	// Enable buffered (old) client for Prometheus datasource as default instead of streaming JSON parser client (new)
-	FlagPrometheusBufferedClient = "prometheusBufferedClient"
-
 	// FlagNewDBLibrary
 	// Use jmoiron/sqlx rather than xorm for a few backend services
 	FlagNewDBLibrary = "newDBLibrary"