af_channelmap: remove stray enum declaration

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavfilter/af_channelmap.c

@@ -124,7 +124,6 @@ static av_cold int channelmap_init(AVFilterContext *ctx, const char *args)
     ChannelMapContext *s = ctx->priv;
     int ret;
     char *mapping;
-    enum mode;
     int map_entries = 0;
     char buf[256];
     enum MappingMode mode;