sh: 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.

[ Please test this on your arch ]

Cc: Matt Fleming <matt@console-pimps.org>
Cc: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>