[NET] front: Remove tx_full and unnecessary queue operations

[NET] front: Remove tx_full and unnecessary queue operations

The tx_full variable merely mirrors information already present in
the XOFF bit on the net device.  The net device architecture itself
is quite mature and can be trusted by Xen to maintain its state
correctly.

Also, it's pointless to stop the queue in close_netdev since it can
be waken up anyway since there could be a softirq running on another
CPU.  All of this is handled by unregister_netdev anyway.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>