chore(tgrid): remove unused imports in cell merging custom strat sample

Author: IMinchev64

samples/grids/tree-grid/cell-merge-custom-sample/src/index.tsx

@@ -1,4 +1,4 @@
-import React, { useEffect, useState } from "react";
+import React, { useState } from "react";
 import ReactDOM from "react-dom/client";
 import "./index.css";