kernel/int: Remove now-unneded ifdef __riscv.

Following the rework to permit arch_int_assign_cpu to change what
CPU an interrupt is assigned to, this ifdef is no longer needed.

src/system/kernel/int.cpp

@@ -411,11 +411,6 @@ restore_interrupts(cpu_status status)
 static
 uint32 assign_cpu(void)
 {
-// arch_int_assign_to_cpu is not yet implemented for riscv
-#ifdef __riscv
-	return 0;
-#endif
-
 	const cpu_topology_node* node;
 	do {
 		int32 nextID = atomic_add(&sLastCPU, 1);