mpeg1videoenc: disable slice threads

It doesnt work (and as far as i tested also didnt in the past)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpeg12enc.c

@@ -974,7 +974,7 @@ AVCodec ff_mpeg1video_encoder = {
     .close          = MPV_encode_end,
     .supported_framerates= avpriv_frame_rate_tab+1,
     .pix_fmts= (const enum PixelFormat[]){PIX_FMT_YUV420P, PIX_FMT_NONE},
-    .capabilities= CODEC_CAP_DELAY | CODEC_CAP_SLICE_THREADS,
+    .capabilities= CODEC_CAP_DELAY,
     .long_name= NULL_IF_CONFIG_SMALL("MPEG-1 video"),
     .priv_class     = &mpeg1_class,
 };