virtio-net: remove unused delayed refill worker

Since we switched to retry refilling receive buffer in NAPI poll instead
of delayed worker, remove all now unused delayed refill worker code.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
Link: https://patch.msgid.link/20260106150438.7425-3-minhquangbui99@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>