providers/proxy: fixed usage of wrong mem ctx
    
    Temporary var `grp` in proxy_id.c:remove_duplicate_group_members()
    should be created in `tmp_ctx`.
    Call to
    ```
    *_grp = talloc_steal(mem_ctx, grp);
    ```
    as well confirms it was original intent
    (before fix this call didn't have any sense).
    
    Having `grp` created in `mem_ctx` may lead to memory leak in case
    of failure. While actually this doesn't happen since caller of
    remove_duplicate_group_members() cleans mem_ctx, still it is
    good to fix it.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>