virtio_net: rx remove premapped failover code

Now, the premapped mode can be enabled unconditionally.

So we can remove the failover code for merge and small mode.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Larysa Zaremba <larysa.zaremba@intel.com>
Link: https://lore.kernel.org/r/20240511031404.30903-4-xuanzhuo@linux.alibaba.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>