tracing: Do not show filter file for ftrace internal events

The filter file in the ftrace internal events, like in
/sys/kernel/tracing/events/ftrace/function/filter is not attached to any
functionality. Do not create them as they are meaningless.

In the future, if an ftrace internal event gets filter functionality, then
it will need to create it directly.

Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>