powerpc: Drop dependency between asm/irq.h and linux/irqdomain.h

Directly including linux/irqdomain.h was hiding all sort of sins,
which have now been fixed. Drop the spurious include.
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/powerpc/include/asm/irq.h

@@ -6,7 +6,6 @@
 /*
  */
 
-#include <linux/irqdomain.h>
 #include <linux/threads.h>
 #include <linux/list.h>
 #include <linux/radix-tree.h>