Undeprecate IAM factory helpers #100

In addition to deprecating legacy role assignments, fd47fda (googleapis/google-cloud-python#9869) deprecated the Policy.user, Policy.service_account, Policy.group, Policy.domain, Policy.all_users, and Policy.authenticated_users entity factory helpers.

ISTM that those helpers should not be deprecated: they hide spelling details from users, and were not part of the "binding assigments" bit being deprecated (the assignable Policy.owners, Policy.editors, Policy.viewers properties): one still has to be able to construct the correctly-spelled entity when using the expected Policy.bindings[<ROLENAME>] spelling.