[source] Drop source mapping handling
It was only used by the cwxmlparser which has been deleted. This is too complex
for litle benefit, we don't want to maintain that in cubicweb.
for rtype in ('cw_support', 'cw_dont_cross', 'cw_may_cross'):
drop_relation_type(rtype)
if not 'url' in schema['CWSource'].subjrels:
add_attribute('CWSource', 'url')
add_attribute('CWSource', 'parser')
add_attribute('CWSource', 'latest_retrieval')