virtio-blk: Remove useless condition around g_free()

Laszlo spotted and studied this wasteful "if". He pointed out:

The original virtio_blk_free_request needed an "if" as it accesses one
field, since 671ec3f0 ("virtio-blk: Convert VirtIOBlockReq.elem to
pointer", 2014-06-11); later on in f897bf75 ("virtio-blk: embed
VirtQueueElement in VirtIOBlockReq", 2014-07-09) the field became
embedded, so the "if" became unnecessary (at which point we were using
g_slice_free(), but it is the same.

Now drop it.
Reported-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/block/virtio-blk.c

@@ -42,9 +42,7 @@ static void virtio_blk_init_request(VirtIOBlock *s, VirtQueue *vq,
 
 static void virtio_blk_free_request(VirtIOBlockReq *req)
 {
-    if (req) {
-        g_free(req);
-    }
+    g_free(req);
 }
 
 static void virtio_blk_req_complete(VirtIOBlockReq *req, unsigned char status)