[media] pixfmt.rst: drop V4L2_YCBCR_ENC_SYCC from the documentation

The V4L2_YCBCR_ENC_SYCC encoding is identical to V4L2_YCBCR_ENC_601.
Remove V4L2_YCBCR_ENC_SYCC from the documentation since it should not
be used.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>