imagefreeze: Remove unused but set duration variable

GCC 4.6.x spits warnings about such variable usage.

gst/imagefreeze/gstimagefreeze.c

@@ -744,7 +744,7 @@ gst_image_freeze_src_loop (GstPad * pad)
   GstImageFreeze *self = GST_IMAGE_FREEZE (GST_PAD_PARENT (pad));
   GstBuffer *buffer;
   guint64 offset;
-  GstClockTime timestamp, timestamp_end, duration;
+  GstClockTime timestamp, timestamp_end;
   gint64 cstart, cstop;
   gboolean in_seg, eos;
 
@@ -800,11 +800,9 @@ gst_image_freeze_src_loop (GstPad * pad)
     timestamp_end =
         gst_util_uint64_scale (offset + 1, self->fps_d * GST_SECOND,
         self->fps_n);
-    duration = timestamp_end - timestamp;
   } else {
     timestamp = self->segment.start;
     timestamp_end = GST_CLOCK_TIME_NONE;
-    duration = GST_CLOCK_TIME_NONE;
   }
 
   eos = (self->fps_n == 0 && offset > 0) ||