Remove keyboard navigation from core (#4593)

core/marker_manager.js

@@ -52,6 +52,13 @@ Blockly.MarkerManager = function(workspace){
   this.workspace_ = workspace;
 };
 
+/**
+ * The name of the local marker.
+ * @type {string}
+ * @const
+ */
+Blockly.MarkerManager.LOCAL_MARKER = 'local_marker_1';
+
 /**
  * Register the marker by adding it to the map of markers.
  * @param {string} id A unique identifier for the marker.