ArmVirtPkg: remove ArmPlatformSysConfigLib dependency

ArmVirtPkg: remove ArmPlatformSysConfigLib dependency

Now that the PL031 RTC driver library no longer depends on the ARM
platform specific ArmPlatformSysConfigLib, we no longer need to
implement ArmPlatform.h or have a resolution for that library.
This allows us to get rid of a rather dodgy practice of including
platform headers using compiler flags, which is a bad idea at various
levels.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>