Plugins: Remove various custom headers logic (#54146) (#54284)

Removes various custom headers logic sprinkled around in the backend.
It should automatically be applied to outgoing HTTP requests via the
CustomHeadersMiddleware.
This also removes decryption of SecureJSONData to populate custom
headers in ngalert which seemed to have caused a ton of CPU usage.

(cherry picked from commit 87afd9cadc)

pkg/services/query/query.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"fmt"
 	"net/http"
-	"strings"
 	"time"
 
 	"github.com/grafana/grafana/pkg/api/dtos"
@@ -26,11 +25,6 @@ import (
 	"github.com/grafana/grafana-plugin-sdk-go/backend/httpclient"
 )
 
-const (
-	headerName  = "httpHeaderName"
-	headerValue = "httpHeaderValue"
-)
-
 func ProvideService(
 	cfg *setting.Cfg,
 	dataSourceCache datasources.CacheService,
@@ -184,10 +178,6 @@ func (s *Service) handleQueryData(ctx context.Context, user *models.SignedInUser
 		}
 	}
 
-	for k, v := range customHeaders(ds.JsonData, instanceSettings.DecryptedSecureJSONData) {
-		req.Headers[k] = v
-	}
-
 	if parsedReq.httpRequest != nil {
 		proxyutil.ClearCookieHeader(parsedReq.httpRequest, ds.AllowedCookies())
 		if cookieStr := parsedReq.httpRequest.Header.Get("Cookie"); cookieStr != "" {
@@ -215,26 +205,6 @@ type parsedRequest struct {
 	httpRequest   *http.Request
 }
 
-func customHeaders(jsonData *simplejson.Json, decryptedJsonData map[string]string) map[string]string {
-	if jsonData == nil {
-		return nil
-	}
-
-	data := jsonData.MustMap()
-
-	headers := map[string]string{}
-	for k := range data {
-		if strings.HasPrefix(k, headerName) {
-			if header, ok := data[k].(string); ok {
-				valueKey := strings.ReplaceAll(k, headerName, headerValue)
-				headers[header] = decryptedJsonData[valueKey]
-			}
-		}
-	}
-
-	return headers
-}
-
 func (s *Service) parseMetricRequest(ctx context.Context, user *models.SignedInUser, skipCache bool, reqDTO dtos.MetricRequest) (*parsedRequest, error) {
 	if len(reqDTO.Queries) == 0 {
 		return nil, NewErrBadQuery("no queries found")