Stop setting a backref on relations that are viewonly
    
    From sqlalachemy 1.3.14 relations that are specifying a backref when
    viewonly=True will issue a warning and this will be disallowed in
    future releases of sqlalchemy.
    So, after checking that these backref were not used in the project,
    we have simply removed them.
    
    Source: https://github.com/sqlalchemy/sqlalchemy/releases/tag/rel_1_3_14
    which also explains why this behavior is not desired.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>