x86/ftrace: Remove unsued extern declaration ftrace_regs_caller_ret()

This is now unused, so can remove it.

Link: https://lore.kernel.org/linux-trace-kernel/20230623091640.21952-1-yuehaibing@huawei.com

Cc: <mark.rutland@arm.com>
Cc: <tglx@linutronix.de>
Cc: <mingo@redhat.com>
Cc: <bp@alien8.de>
Cc: <dave.hansen@linux.intel.com>
Cc: <x86@kernel.org>
Cc: <hpa@zytor.com>
Cc: <peterz@infradead.org>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>