EmbeddedPkg/PrePiLib: Drop unused PCD PcdPrePiCpuIoSize

EmbeddedPkg/PrePiLib: Drop unused PCD PcdPrePiCpuIoSize

The code never references the PCD so the INF should not reference it
either. The last reference was moved to PrePiHobLib in commit
05e48e00834b32e4bd2 (dated 2011!) and dropped from there as well in
commit 97106391955337155e7.

Cc: Leif Lindholm <quic_llindhol@quicinc.com>
Cc: Abner Chang <abner.chang@amd.com>
Cc: Daniel Schaefer <git@danielschaefer.me>
Reported-by: Sunil V L <sunilvl@ventanamicro.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>