Fix T97744: Speed effect causes memory leak

Leak was caused because output image buffer was initialized twice. Once
in speed effect strip and then by cross effect strip used for
interpolation feature.

source/blender/sequencer/intern/effects.c

@@ -2712,7 +2712,6 @@ static ImBuf *do_speed_effect(const SeqRenderData *context,
   ImBuf *out;
 
   if (s->flags & SEQ_SPEED_USE_INTERPOLATION) {
-    out = prepare_effect_imbufs(context, ibuf1, ibuf2, ibuf3);
     fac = speed_effect_interpolation_ratio_get(context->scene, seq, timeline_frame);
     /* Current frame is ibuf1, next frame is ibuf2. */
     out = seq_render_effect_execute_threaded(