references to clear_mapper still exist

[Former user]

In orm.mapper.py, the following reference to clear_mapper still exists: raise exceptions.ArgumentError("Class '%s' already has a primary mapper defined with entity name '%s'. Use non_primary=True to create a non primary Mapper, or to create a new primary mapper, remove this mapper first via sqlalchemy.orm.clear_mapper(mapper), or preferably sqlalchemy.orm.clear_mappers() to clear all mappers." % (self.class_, self.entity_name))

This causes confusion on the part of the user when trying to load a function that no longer exists!

[Mike Bayer]

f3af574650d19fb2b010ef5b42e0a3e68ed8cf61