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_test.go

@@ -2,7 +2,6 @@ package query_test
 
 import (
 	"context"
-	"encoding/json"
 	"net/http"
 	"testing"
 
@@ -25,22 +24,6 @@ import (
 )
 
 func TestQueryData(t *testing.T) {
-	t.Run("it attaches custom headers to the request", func(t *testing.T) {
-		tc := setup(t)
-		tc.dataSourceCache.ds.JsonData = simplejson.NewFromAny(map[string]interface{}{"httpHeaderName1": "foo", "httpHeaderName2": "bar"})
-
-		secureJsonData, err := json.Marshal(map[string]string{"httpHeaderValue1": "test-header", "httpHeaderValue2": "test-header2"})
-		require.NoError(t, err)
-
-		err = tc.secretStore.Set(context.Background(), tc.dataSourceCache.ds.OrgId, tc.dataSourceCache.ds.Name, "datasource", string(secureJsonData))
-		require.NoError(t, err)
-
-		_, err = tc.queryService.QueryData(context.Background(), nil, true, metricRequest(), false)
-		require.Nil(t, err)
-
-		require.Equal(t, map[string]string{"foo": "test-header", "bar": "test-header2"}, tc.pluginContext.req.Headers)
-	})
-
 	t.Run("it auth custom headers to the request", func(t *testing.T) {
 		token := &oauth2.Token{
 			TokenType:   "bearer",