avcodec/mpegvideo_enc: Remove slice structured mode from H.263 as well as the code automatically enabing it

There is no such thing as a slice structured mode in the original version 1  H.263,
that mode was added in H.263+ in 1998. Also the headers for slice structured mode
are not part of the older version 1 and this would result in unplayable files

An alternative to this patch would be to merge the H263 and H263P AVCodecs and use
other means to distinguish the older and newer versions.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>