[IA64] remove CONFIG_IA64_SGI_SN_SIM

This patch removes the CONFIG_IA64_SGI_SN_SIM option entirely, allowing
any kernel bootable on sn2 to also be booted in the simulator.

Boot tested on Altix and HP rx2600.

Signed-off-by: Greg Edwards <edwardsg@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>