openrisc/traps: Remove die_if_kernel function

This was noticed when I saw this warning:

    arch/openrisc/kernel/traps.c:234:6: warning: no previous prototype for 'die_if_kernel' [-Wmissing-prototypes]
      234 | void die_if_kernel(const char *str, struct pt_regs *regs, long err)
	  |      ^~~~~~~~~~~~~

The die_if_kernel function is not used in the OpenRISC port so remove
it.

Signed-off-by: Stafford Horne <shorne@gmail.com>