powerpc/4xx: Remove pika_dtm_[un]register_shutdown() to fix no previous prototype

ppc4xx_defconfig with W=1 results in:

  CC      arch/powerpc/platforms/44x/warp.o
arch/powerpc/platforms/44x/warp.c:369:5: error: no previous prototype for 'pika_dtm_register_shutdown' [-Werror=missing-prototypes]
  369 | int pika_dtm_register_shutdown(void (*func)(void *arg), void *arg)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~
arch/powerpc/platforms/44x/warp.c:374:5: error: no previous prototype for 'pika_dtm_unregister_shutdown' [-Werror=missing-prototypes]
  374 | int pika_dtm_unregister_shutdown(void (*func)(void *arg), void *arg)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

The functions were added by commit 4ebef31f ("[POWERPC] PIKA Warp:
Update platform code to support Rev B boards")

Those functions are not used localy and allthough their symbols are
exported they are not declared in any header file so they can't be used.

Remove them, then remove the associated list as it will now remain empty
hence becomes useless.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/830923f0e0375a14609204246d302c7476a8f948.1692279855.git.christophe.leroy@csgroup.eu