Platform/AMD/OverdriveBoard: drop DO_XGBE C preprocessor defines

Now that all code relies on the PCD rather than the DO_XGBE macro,
get rid of it.

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