powerpc/eeh: remove unused macro IS_BRIDGE

Currently, the macro IS_BRIDGE is not used any where.
This patch just removes it.

Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Acked-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>