tcp: md5: hide unused variable

Changing from a memcpy to per-member comparison left the
size variable unused:

net/ipv4/tcp_ipv4.c: In function 'tcp_md5_do_lookup':
net/ipv4/tcp_ipv4.c:910:15: error: unused variable 'size' [-Werror=unused-variable]

This does not show up when CONFIG_IPV6 is enabled, but the
variable can be removed either way, along with the now unused
assignment.

Fixes: 6797318e ("tcp: md5: add an address prefix for key lookup")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>