tracing: Remove orphaned function using_ftrace_ops_list_func().

Remove using_ftrace_ops_list_func() since it is no longer used.

Using ftrace_ops_list_func() has been introduced by commit 7eea4fce
("tracing/stack_trace: Skip 4 instead of 3 when using ftrace_ops_list_func")
as a helper function, but its caller has been removed by commit 72ac426a


("tracing: Clean up stack tracing and fix fentry updates").  So it is not
called anymore.

Link: http://lkml.kernel.org/r/153260904427.12474.9952096317439329851.stgit@devbox

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>