sfc: remove unneeded variable

We don't use ->heap_buf after commit 46d1efd8 ("sfc: remove Software
TSO") so let's remove the last traces.

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