hw/net: Remove unnecessary header includes

Headers like "hw/loader.h" and "qemu/sockets.h" are not needed in
the hw/net/*.c files. And Some other headers are included via other
headers already, so we can drop them, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>