shell-recorder: Timestamp buffers from the pipeline clock, not the wall clock

This should give more accurate timestamps on the video buffers, and especially
keep A/V sync when audio is also recorded.

https://bugzilla.gnome.org/show_bug.cgi?id=744642

src/shell-recorder-src.c

@@ -43,7 +43,6 @@ shell_recorder_src_init (ShellRecorderSrc      *src)
 {
   gst_base_src_set_format (GST_BASE_SRC (src), GST_FORMAT_TIME);
   gst_base_src_set_live (GST_BASE_SRC (src), TRUE);
-  gst_base_src_set_do_timestamp (GST_BASE_SRC (src), TRUE);
 
   src->queue = g_async_queue_new ();
   src->mutex = &src->mutex_data;