genirq: remove irq_to_desc_alloc

Remove the leftover of sparseirqs.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
diff --git a/arch/x86/kernel/io_apic.c b/arch/x86/kernel/io_apic.c
index 6836105..e03bc0f 100644
--- a/arch/x86/kernel/io_apic.c
+++ b/arch/x86/kernel/io_apic.c
@@ -1257,11 +1257,7 @@
 {
 	struct irq_desc *desc;
 
-	/* first time to use this irq_desc */
-	if (irq < 16)
-		desc = irq_to_desc(irq);
-	else
-		desc = irq_to_desc_alloc(irq);
+	desc = irq_to_desc(irq);
 
 	if ((trigger == IOAPIC_AUTO && IO_APIC_irq_trigger(irq)) ||
 	    trigger == IOAPIC_LEVEL)