lavfi/sr: Remove slice thread flag

sr didn't enable the slice threading, so remove the flag
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

libavfilter/vf_sr.c

@@ -317,6 +317,5 @@ AVFilter ff_vf_sr = {
     .inputs        = sr_inputs,
     .outputs       = sr_outputs,
     .priv_class    = &sr_class,
-    .flags         = AVFILTER_FLAG_SUPPORT_TIMELINE_GENERIC | AVFILTER_FLAG_SLICE_THREADS,
+    .flags         = AVFILTER_FLAG_SUPPORT_TIMELINE_GENERIC,
 };
-