Platform/SoftIron/Overdrive1000Board: remove dead XGBE references

Remove the references to the XGBE SNP drivers, which are inside a
!ifdef() that never gets enabled anyway.

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