powerpc: Remove unused SLOW_DOWN_IO definition

Remove unused SLOW_DOWN_IO definition.
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220415190817.842864-6-helgaas@kernel.org

arch/powerpc/include/asm/io.h

@@ -38,8 +38,6 @@ extern struct pci_dev *isa_bridge_pcidev;
 #define SIO_CONFIG_RA	0x398
 #define SIO_CONFIG_RD	0x399
 
-#define SLOW_DOWN_IO
-
 /* 32 bits uses slightly different variables for the various IO
  * bases. Most of this file only uses _IO_BASE though which we
  * define properly based on the platform