ServiceManagerNative: delete unused methods

This class is never instantiated (and should be deleted in the future),
doesn't need to inherit from anything, and only has one method that is
used (to get an instance of BpServiceManager defined here). Deleting
unused code, and we can delete the class once apps stop linking against
it.

Bug: 135768100
Test: TH
Change-Id: Ie19e099551b10599a4b7bab29b35d78d66cd3442