ftrace: Remove unused ftrace_force_update()

ftrace_force_update() is committed by Commit e1c08bdd ("ftrace: force
recording") and removed by Commit cb7be3b2 ("ftrace: remove daemon").
Remove it in header file.

Link: https://lkml.kernel.org/r/1612409671-8249-1-git-send-email-hejinyang@loongson.cn



Signed-off-by: Jinyang He <hejinyang@loongson.cn>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>