lavfi/showspatial: Remove two write-only variables.

Fixes two warnings:
libavfilter/avf_showspatial.c:157:26: warning: variable ‘w’ set but not used
libavfilter/avf_showspatial.c:157:23: warning: variable ‘h’ set but not used

libavfilter/avf_showspatial.c

@@ -154,16 +154,13 @@ static int config_output(AVFilterLink *outlink)
     AVFilterContext *ctx = outlink->src;
     AVFilterLink *inlink = ctx->inputs[0];
     ShowSpatialContext *s = ctx->priv;
-    int i, fft_bits, h, w;
+    int i, fft_bits;
     float overlap;
 
     outlink->w = s->w;
     outlink->h = s->h;
     outlink->sample_aspect_ratio = (AVRational){1,1};
 
-    h = s->h;
-    w = s->w;
-
     s->buf_size = 1 << av_log2(s->win_size);
     s->win_size = s->buf_size;
     fft_bits = av_log2(s->win_size);