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/tsdb/prometheus/middleware/req_headers.go

@@ -1,24 +0,0 @@
-package middleware
-
-import (
-	"net/http"
-
-	sdkHTTPClient "github.com/grafana/grafana-plugin-sdk-go/backend/httpclient"
-)
-
-// ReqHeadersMiddleware is used so that we can pass req headers through the prometheus go client as it does not allow
-// access to the request directly. Should be used together with WithContextualMiddleware so that it is attached to
-// the context of each request with its unique headers.
-func ReqHeadersMiddleware(headers map[string]string) sdkHTTPClient.Middleware {
-	return sdkHTTPClient.NamedMiddlewareFunc("prometheus-req-headers-middleware", func(opts sdkHTTPClient.Options, next http.RoundTripper) http.RoundTripper {
-		return sdkHTTPClient.RoundTripperFunc(func(req *http.Request) (*http.Response, error) {
-			for k, v := range headers {
-				// As custom headers middleware is before contextual we may overwrite custom headers here with those
-				// that came with the request which probably makes sense.
-				req.Header[k] = []string{v}
-			}
-
-			return next.RoundTrip(req)
-		})
-	})
-}