tracing/events: make SAMPLE_TRACE_EVENTS default to n

Normally a config should be default to n. This patch also makes the
sample module-only, like SAMPLE_MARKERS and SAMPLE_TRACEPOINTS.

[ Impact: don't build trace event sample by default ]

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <4A00F6C0.8090803@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>