Remove useless if statement in ResourceTestBase::assertResourceResponse()

Marking as 'Novice'.

It looks this problem was introduced in #2805281-10: ?_format=hal_json error responses are application/json, yet should be application/hal+json

The previous block was

if ($expected_status_code < 400) {
      $this->assertSame([static::$mimeType], $response->getHeader('Content-Type'));
    }
    else {
      $this->assertSame([static::$expectedErrorMimeType], $response->getHeader('Content-Type'));
    }

But then in that issue static::$expectedErrorMimeType was no longer needed. So maybe just replace error.

Setting back to needs review. #2 had a totally unrelated error. Not sure why

@Wim Leers yep sorry I added the tag because I thought a novice could mark it RTBC.