Remove unused imported function and interface (#259)

Also the function `GuzzleHttp\Psr7\str` is remoced from `guzzle/psr7` v2.0

Author: 10n

src/Common/Http/Client.php

@@ -2,7 +2,6 @@
 
 namespace Omnipay\Common\Http;
 
-use function GuzzleHttp\Psr7\str;
 use Http\Client\HttpClient;
 use Http\Discovery\HttpClientDiscovery;
 use Http\Discovery\MessageFactoryDiscovery;
@@ -12,7 +11,6 @@
 use Psr\Http\Message\RequestInterface;
 use Psr\Http\Message\ResponseInterface;
 use Psr\Http\Message\StreamInterface;
-use Psr\Http\Message\UriInterface;
 
 class Client implements ClientInterface
 {