firewire: net: remove unused variable 'guid'

GCC reports the following warning with W=1:

drivers/firewire/net.c:493:9: warning:
 variable ‘guid’ set but not used [-Wunused-but-set-variable]
  493 |  __be64 guid;
      |         ^~~~

This variable is not used anymore since commit 6752c8db ("firewire
net, ipv4 arp: Extend hardware address and remove driver-level packet
inspection."). Remove it to fix the warning.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>