ax25: remove unneeded NULL test in ax_xmit()

We get a static checker warning here on devel kernels:

	drivers/net/hamradio/mkiss.c:560 ax_xmit()
	warn: variable dereferenced before check 'skb' (see line 532)

It turns out that the NULL check can be deleted.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>