ArmPlatformPkg/PL031RealTimeClockLib: drop ArmPlatformSysConfigLib reference

ArmPlatformPkg/PL031RealTimeClockLib: drop ArmPlatformSysConfigLib reference

The PL031 driver implements a VExpress/Juno specific hack to set the
battery backed clock in addition to the PL031. However, none of the
remaining VExpress based hardware we support in EDK2 actuall implements
this feature so we can just remove it, and get rid of the cumbersome
dependency on ArmPlatform.h.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>