rgw/services: don't expose RGWServices::notify

rgw/services: don't expose RGWServices::notify

RGWSI_Notify is only used internally by RGWSI_SysObj_Cache where the
dependency is injected to RGWSI_SysObj_Cache::init(). there's no need to
expose a pointer to RGWSI_Notify to the rest of the application,
especially now that it can be a nullptr

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit dfc379ccf31cb2b4a68d123af8af70b7aae0b377)