fix(query-builder): Removed unused refs

Author: ivanvpetrov

projects/igniteui-angular/src/lib/query-builder/query-builder-tree.component.ts

@@ -1,9 +1,7 @@
 import {
     AfterViewInit,
     ContentChild,
-    ContentChildren,
     EventEmitter,
-    forwardRef,
     LOCALE_ID,
     Output,
     Pipe,

src/app/query-builder/query-builder.sample.ts

@@ -1,4 +1,4 @@
-import { Component, ViewChild, OnInit, TemplateRef } from '@angular/core';
+import { Component, ViewChild, OnInit } from '@angular/core';
 import {
     FilteringExpressionsTree, IgxStringFilteringOperand,
     FilteringLogic,