tap: Drop tap_can_send

This callback is called by main loop before polling s->fd, if it returns
false, the fd will not be polled in this iteration.

This is redundant with checks inside read callback. After this patch,
the data will be sent to peer when it arrives. If the device can't
receive, it will be queued to incoming_queue, and when the device status
changes, this queue will be flushed.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1433400324-7358-7-git-send-email-famz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
