tracing: Consolidate ftrace_trace_onoff_unreg() into callback

The only thing ftrace_trace_onoff_unreg() does is to do a strcmp()
against the cmd parameter to determine what op to unregister. But
this compare is also done after the location that this function is
called (and returns). By moving the check for '!' to unregister after
the strcmp(), the callback function itself can just do the unregister
and we can get rid of the helper function.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>