Blackfin: ftrace: Remove check of obsolete variable function_trace_stop

Nothing sets function_trace_stop to disable function tracing anymore.
Remove the check for it in the arch code.

Link: http://lkml.kernel.org/r/3144266.ziutPk5CNZ@vapier



Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>