ArmPlatformPkg: remove obsolete/outdated AArch64 bootstrap

Harking back to the earliest days of software development for AArch64,
this piece of code abstracted away "early setup stuff" when the
software model came out of reset.
However, it is unmaintained and has been superseded by ARM Trusted
Firmware. So drop it.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Ryan Harkin <ryan.harkin@linaro.org>