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>