lavfi/aevalsrc: do not free AVOption variables in uninit()

The generic code frees them already.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/asrc_aevalsrc.c

@@ -150,8 +150,6 @@ static av_cold void uninit(AVFilterContext *ctx)
         av_expr_free(eval->expr[i]);
         eval->expr[i] = NULL;
     }
-    av_freep(&eval->chlayout_str);
-    av_freep(&eval->sample_rate_str);
 }
 
 static int config_props(AVFilterLink *outlink)