powerpc/configs/6xx: Drop removed CONFIG_BLK_DEV_HD

In commit 8e14be53 ("remove the obsolete hd driver") the
CONFIG_BLK_DEV_HD symbol was removed, so drop it from the defconfig.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/configs/ppc6xx_defconfig

@@ -368,7 +368,6 @@ CONFIG_BLK_DEV_RAM=y
 CONFIG_BLK_DEV_RAM_SIZE=16384
 CONFIG_CDROM_PKTCDVD=m
 CONFIG_VIRTIO_BLK=m
-CONFIG_BLK_DEV_HD=y
 CONFIG_ENCLOSURE_SERVICES=m
 CONFIG_SENSORS_TSL2550=m
 CONFIG_EEPROM_AT24=m