chore: remove next-runtime-env import from GrowthBook documentation

Signed-off-by: SeSo <sobhani.sepehr@gmail.com>

Author: Sepehr-Sobhani

docs/frontend/growthbook.md

@@ -32,7 +32,6 @@ import {
   GrowthBookProvider
 } from "@growthbook/growthbook-react";
 import { useMemo, useEffect, useState } from "react";
-import { env } from 'next-runtime-env';
 
 const CLIENT_KEY = env('NEXT_PUBLIC_GROWTHBOOK_CLIENT_KEY')
 const growthbook_clientPage = new GrowthBook({