When compiling with W=1 enabling -Wmissing-prototypes the compiler
warns:

  arch/openrisc/kernel/traps.c:146:6: error: no previous prototype for 'nommu_dump_state' [-Werror=missing-prototypes]

This function is not used so remove it.
Reported-by: Arnd Bergmann <arnd@arndb.de>
Closes: https://lore.kernel.org/linux-kernel/20230810141947.1236730-17-arnd@kernel.org/Signed-off-by: Stafford Horne <shorne@gmail.com>