powerpc/powernv: remove orphan declarations from opal.h

Remove the following orphan declarations from opal.h:
1. opal_notifier_register()
2. opal_notifier_unregister()
3. opal_notifier_update_evt()
4. opal_notifier_enable()
5. opal_notifier_disable()

They have been removed since commit 81f2f7ce ("opal:
Remove events notifier"), so remove them.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220913075029.682327-7-cuigaosheng1@huawei.com