Platform/RISC-V: Remove Null instance of OpensbiPlatformLibNull

Remove Null instance of OpensbiPlatformLibNull,
OpensbiPlatformLib is the generic one for RsicVPlatformPkg.

Cc: Sunil V L <sunilvl@ventanamicro.com>
Cc: Daniel Schaefer <daniel.schaefer@hpe.com>
Signed-off-by: Abner Chang <abner.chang@hpe.com>
Reviewed-by: Daniel Schaefer <daniel.schaefer@hpe.com>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>