Issue Summary

users are unable to delete object type attributes when they have set up multiple import configurations that are associated with that same attribute.

Steps to Reproduce

1. Set up 2x import configurations so that they map to the same attribute/object type
2. Try to delete that attribute from above from its object type

Expected Results

User can delete the attribute

Actual Results

The user cannot delete the attribute and is faced with a 500 error

Workaround

You can remove one of the attribute mappings from one of the configurations. 
Alternatively you could delete the whole import configuration.
Just make sure that there's only one mapping for an attribute.