Hisilicon: drop unused DS3231RealTimeClockLib instance

Edk2-platforms contains two DS3231RealTimeClockLib instances:

- Silicon/Hisilicon/Library/DS3231RealTimeClockLib
- Platform/Hisilicon/D03/Library/DS3231RealTimeClockLib

The latter is still used (by the D03 and D05 platforms), but the former
has not been referenced since commit 478282ea ("Hisilicon/D02: Remove
D02 platform", 2018-08-08). Remove the unused library instance.

(There's no reference in edk2-non-osi either.)

Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Wenyi Xie <xiewenyi2@huawei.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=4564


Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>