rxrpc: remove redundant static int 'zero'

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: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>