[ML] Hide selector helper in Anomaly Explorer swimlane (#65522)

Author: darnautov

x-pack/plugins/ml/public/application/explorer/_explorer.scss

@@ -1,3 +1,7 @@
+.ml-swimlane-selector {
+  visibility: hidden;
+}
+
 .ml-explorer {
   width: 100%;
   display: inline-block;

x-pack/plugins/ml/public/application/explorer/explorer.js

@@ -120,6 +120,7 @@ export class Explorer extends React.Component {
   disableDragSelectOnMouseLeave = true;
 
   dragSelect = new DragSelect({
+    selectorClass: 'ml-swimlane-selector',
     selectables: document.getElementsByClassName('sl-cell'),
     callback(elements) {
       if (elements.length > 1 && !ALLOW_CELL_RANGE_SELECTION) {