tests: remove LOCAL_SYSDB_FILE reference from common_dom
    
    As all tests are now taking advantage of the "files" provider instead of
    the "local" one, let's just remove the last reference of
    LOCAL_SYSDB_FILE from our tests.
    
    Together with the reference, let's also remove the whole if-block as
    we're not relying on "local" provider anymore.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3304
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>