x86/platform/iosf_mbi: Remove unused iosf_mbi_unregister_pmic_bus_access_notifier()

The last use of iosf_mbi_unregister_pmic_bus_access_notifier() was
removed in 2017 by:

  a5266db4 ("drm/i915: Acquire PUNIT->PMIC bus for intel_uncore_forcewake_reset()")

Remove it.

(Note that the '_unlocked' version is still used.)

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tursulin@ursulin.net>
Cc: David Airlie <airlied@gmail.com>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Link: https://lore.kernel.org/r/20241225175010.91783-1-linux@treblig.org