powerpc/ps3: Remove unused os_area_db_id_video_mode

This struct is unused, which is now a build error with gcc 6:

  error: 'os_area_db_id_video_mode' defined but not used

There doesn't seem to be any good reason to keep it around so remove it,
it's in the history if anyone needs it.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>