lavc/h264dec.h: remove unused ff_h264_decode_slice_header

Once removed in 4a9bab3d.

Introduced again in b25cd754.
Signed-off-by: Linjie Fu <fulinjie@zju.edu.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264dec.h

@@ -832,8 +832,6 @@ int ff_h264_slice_context_init(H264Context *h, H264SliceContext *sl);
 
 void ff_h264_draw_horiz_band(const H264Context *h, H264SliceContext *sl, int y, int height);
 
-int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl,
-                                const H2645NAL *nal);
 /**
  * Submit a slice for decoding.
  *