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 commit 80006dbe ("kprobes/x86:
Remove jprobe implementation") has removed the caller in kprobes.
So this doesn't exist anymore.

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