virtio-blk: Remove useless condition around g_free()

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 671ec3f05655 ("virtio-blk: Convert VirtIOBlockReq.elem to
pointer", 2014-06-11); later on in f897bf751fbd ("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: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>