The static int 'zero' is defined but is never used hence it is
redundant and can be removed. The use of this variable was removed
with commit a158bdd3 ("rxrpc: Fix call timeouts").

Cleans up clang warning:
warning: 'zero' defined but not used [-Wunused-const-variable=]
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>