clear_ftrace_function is not used outside of ftrace.c and is not help to
use a function, so nuke it per Steve's suggestion.

Link: http://lkml.kernel.org/r/1517537689-34947-1-git-send-email-xieyisheng1@huawei.comSuggested-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NYisheng Xie <xieyisheng1@huawei.com>
Signed-off-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>