rxrpc: remove rxrpc_max_call_lifetime declaration

rxrpc_max_call_lifetime has been removed since
commit a158bdd3 ("rxrpc: Fix call timeouts"),
so remove it.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Link: https://lore.kernel.org/r/20220909064042.1149404-1-cuigaosheng1@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>