kprobes: Remove show_registers() function prototype

Remove show_registers() function prototype because this function
has been renamed by commit:

  57da8b96 ("x86: Avoid double stack traces with show_regs()")

and this commit has removed the caller in kprobes altogether:

  80006dbe ("kprobes/x86: Remove jprobe implementation")

So this doesn't exist anymore - remove the orphan prototype.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>