Remove legacy omx_message variables

Remove platform_private and data_ptr from omx_message.
These were used for legacy deferred buffer creation.
See OMXCodec quirks: kDefersOutputBufferAllocation and
kRequiresAllocateBufferOnOutputPorts.

Change-Id: I94da56bc64f3b46ee1276e93d67b36c1662e2eaf
Signed-off-by: Andy Hung <hunga@google.com>