doc: Remove text on tracepoint samples

The tracepoint sample code got removed. Remove a few lines on its usage
too.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Rob Landley <rob@landley.net>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>