Remove data pointer field from gst_flow_data and gst_flow_event LTTng tracepoints.

LTTng tracepoints gst_flow_data and gst_flow_event only make sense
with the immediately following gst_flow tracepoint on the same thread,
which already contains the pointer to data.

Signed-off-by: Alexander Barabash <alexander_barabash@mentor.com>