* glom/base_db.[h|cc]: Move remove_user() and
remove_user_from_group() to
* glom/libglom/db_utils.[h|cc].
* glom/frame_glom.cc:
* glom/mode_design/users/dialog_users_list.cc: Adapted.
* tests/test_selfhosting_new_empty_then_users.cc: Test these
functions.