[media] v4l2-dv-timings.h: remove duplicate V4L2_DV_BT_DMT_1366X768P60

This particular DMT timing definition was duplicated in the header.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>