Elasticsearch: Fix pre-v7.0 and alerting error (#16904)

This fixes a regression introduced in #16646 where using Elasticsearch 
pre-v7.0 and alerting resulted in an error when trying to deserialize the 
response of total number of hits.
Total number of hits is not in use by the backend so we're removing it 
for now to make ES 6 and 7 being able to deserialize search responses 
without errors.

Closes #15622

pkg/tsdb/elasticsearch/client/models.go

@@ -41,8 +41,7 @@ func (r *SearchRequest) MarshalJSON() ([]byte, error) {
 
 // SearchResponseHits represents search response hits
 type SearchResponseHits struct {
-	Hits  []map[string]interface{}
-	Total map[string]interface{}
+	Hits []map[string]interface{}
 }
 
 // SearchResponse represents a search response
@@ -52,21 +51,6 @@ type SearchResponse struct {
 	Hits         *SearchResponseHits    `json:"hits"`
 }
 
-// func (r *Response) getErrMsg() string {
-// 	var msg bytes.Buffer
-// 	errJson := simplejson.NewFromAny(r.Err)
-// 	errType, err := errJson.Get("type").String()
-// 	if err == nil {
-// 		msg.WriteString(fmt.Sprintf("type:%s", errType))
-// 	}
-
-// 	reason, err := errJson.Get("type").String()
-// 	if err == nil {
-// 		msg.WriteString(fmt.Sprintf("reason:%s", reason))
-// 	}
-// 	return msg.String()
-// }
-
 // MultiSearchRequest represents a multi search request
 type MultiSearchRequest struct {
 	Requests []*SearchRequest