plat/sgm: Remove redundant platform_oid.h

This file is used when building the cert_create tool without using
the 'standard' set of Arm OID values as defined in the TBBR
specification (see tbbr_oid.h). This configuration is enabled by
setting USE_TBBR_DEFS to 0 during build.

At the moment this will fail because the header file included by
this file was removed in commit bb41eb7a

 ("cert: move
platform_oid.h to include/tools_share for all platforms"). For
the SGM platform this means that there is no current use for
this file.

Change-Id: I3c82983ada62330f1ab6be6d6c0cf489adabae7b
Signed-off-by: Paul Beesley <paul.beesley@arm.com>

plat/arm/css/sgm/include/platform_oid.h

-/*
- * Copyright (c) 2018, ARM Limited and Contributors. All rights reserved.
- *
- * SPDX-License-Identifier: BSD-3-Clause
- */
-
-#include <plat/arm/board/common/board_arm_oid.h>
-
-/*
- * Required platform OIDs
- * (Provided by included header)
- */