tracing: Kill ftrace_event_call->files

Remove ftrace_event_call->files. It has no users, and in fact even
the commit ae63b31e "tracing: Separate out trace events from
global variables" which added this member did not use it.

Link: http://lkml.kernel.org/p/20140715184827.GA20508@redhat.com



Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>