avfilter/vf_histeq: remove unused item from filter private context

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/vf_histeq.c

@@ -58,7 +58,6 @@ typedef struct {
     float strength;
     float intensity;
     enum HisteqAntibanding antibanding;
-    char* antibanding_str;
     int in_histogram [256];        ///< input histogram
     int out_histogram[256];        ///< output histogram
     int LUT[256];                  ///< lookup table derived from histogram[]