ArmPlatformPkg/Documentation: remove outdated porting manual

ArmPlatformPkg/Documentation: remove outdated porting manual

Having a special porting manual for ARM platforms in general suggests
that ARM platforms are fundamentally different from ones based on other
architectures that are supported by UEFI. There may be some truth to
that, but the porting manual in Documentation/ArmPlatformPkg.txt is
hopelessly outdated, and did not give the best advice in the first
place*. So remove it.

* ArmPlatformLib as the mother of all platform abstractions is rather
  unwieldy, and using ArmVExpressLibCTA9x4 as a template is not that
  great an idea either.

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