Update TreeDraggedElement service to use Angular 6+ providedIn syntax to improve tree-shaking / reduce bundle size

https://angular.io/guide/singleton-services#using-providedin

Author: Keith Gillette

projects/angular-tree-component/src/lib/angular-tree-component.module.ts

@@ -74,12 +74,6 @@ import './polyfills';
   providers: []
 })
 export class TreeModule {
-  static forRoot(): ModuleWithProviders {
-    return {
-      ngModule: TreeModule,
-      providers: [TreeDraggedElement]
-    };
-  }
 }
 
 export {

projects/angular-tree-component/src/lib/models/tree-dragged-element.model.ts

@@ -1,6 +1,8 @@
 import { Injectable } from '@angular/core';
 
-@Injectable()
+@Injectable({
+  providedIn: 'root'
+})
 export class TreeDraggedElement {
   _draggedElement: any = null;