[DX] Mark user_filters() and user_build_filter_query() as deprecated

[avpaderno]

In Drupal 7, user_filters() is called from user_admin(), the page callback for admin/people, via user_filter_form().

In Backdrop, admin/people is a view. user_filters() is not used to build options for a form element that is shown to people with an account on a Backdrop site. It is called only from user_build_filter_query(), which is then only called by database_test_theme_tablesort(), a test function whose purpose is testing theme_tableselect().

database_test_theme_tablesort() should be rewritten to avoid user_build_filter_query(). user_filters() and user_build_filter_query() should be marked deprecated.