app/Http/Middleware/RedirectWwwToNonWww.php

<?php
namespace App\Http\Middleware;

use Closure;

class RedirectWwwToNonWww
{
    /**
     * @param $request
     * @param Closure $next
     * @return mixed
     */
    public function handle($request, Closure $next)
    {
        $hostname = $request->getHttpHost();
        [$subdomain] = explode('.', $hostname);

        return $subdomain === 'www'
            ? redirect(env('APP_URL'))
            : $next($request);
    }
}

app/Http/Kernel.php

<?php
namespace App\Http;

use Illuminate\Foundation\Http\Kernel as HttpKernel;

class Kernel extends HttpKernel
{
    ...
    protected $middleware = [
        ...
        // Register the class as middleware
        \App\Http\Middleware\RedirectWwwToNonWww::class,
    ];
    ...