mb/sapphire/pureplatinum/cmos.default: Remove `volume` entry

Following commit de50d39, remove the `volume` entry from cmos.default.

Change-Id: Ieebafffa0d2fbf6cdd24cff4ddefe090a727cbfa
Signed-off-by: Nicola Corna <nicola@corna.info>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/52027
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/sapphire/pureplatinumh61/cmos.default

@@ -2,6 +2,5 @@ boot_option=Fallback
 debug_level=Debug
 power_on_after_fail=Enable
 nmi=Enable
-volume=0x3
 sata_mode=AHCI
 hyper_threading=Enable