Plugins: Remove various custom headers logic (#54146)

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.

pkg/services/query/query.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"fmt"
 	"net/http"
-	"strings"
 	"time"
 
 	"github.com/grafana/grafana/pkg/api/dtos"
@@ -27,11 +26,6 @@ import (
 	"github.com/grafana/grafana-plugin-sdk-go/backend/httpclient"
 )
 
-const (
-	headerName  = "httpHeaderName"
-	headerValue = "httpHeaderValue"
-)
-
 func ProvideService(
 	cfg *setting.Cfg,
 	dataSourceCache datasources.CacheService,
@@ -185,10 +179,6 @@ func (s *Service) handleQueryData(ctx context.Context, user *user.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 != "" {
@@ -216,26 +206,6 @@ 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 *user.SignedInUser, skipCache bool, reqDTO dtos.MetricRequest) (*parsedRequest, error) {
 	if len(reqDTO.Queries) == 0 {
 		return nil, NewErrBadQuery("no queries found")