tracing: Introduce remove_event_file_dir()

Preparation for the next patch. Extract the common code from
remove_event_from_tracers() and __trace_remove_event_dirs()
into the new helper, remove_event_file_dir().

The patch looks more complicated than it actually is, it also
moves remove_subsystem() up to avoid the forward declaration.

Link: http://lkml.kernel.org/r/20130726172547.GA3629@redhat.com
Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>