tile: 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.

Cc: Chris Metcalf <cmetcalf@tilera.com>
Acked-by: Zhigang <Lu&lt;zlu@tilera.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>