Make DeleteUser call updatePolicyManagerUsers() to refresh the policy
manager's cached user list after user deletion. This ensures consistency
with CreateUser, UpdateUser, and RenameUser which all update the policy
manager.
Previously, DeleteUser only removed the user from the database without
updating the policy manager. This could leave stale user references in
the cached user list, potentially causing issues when policy is
re-evaluated.
The gRPC handler now uses the change returned from DeleteUser instead of
manually constructing change.UserRemoved().
Fixes#2967
4be13baf3f