core.c: Remove subnet_up_delay parameter

If subnet is not yet up, received SSA membership requests are
put into an "orphan" list. The distribution tree is built
after receiving "SUBNET UP" event,

Signed-off-by: Sasha Kotchubievsky <sashakot@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>