Api _pruneAttributes(): only prune the correct entity type.

When I imported attributes and used Danslo_ApiImport_Model_Import::BEHAVIOR_DELETE_IF_NOT_EXIST, it started deleting my user_defined 'Customer' attributes too. This fixes it.

PS awesome code. I need to migrate products+attributes from one store to another one, knowing almost nothing about Magento. I thought I'd go crazy, but this API code guides me exactly where I need to know.

PPS I guess this is the best branch to do the pull request on, because you know the attribute-related code. Then later, danslo will need to merge this branch which he already rebased earlier... but I guess that will go OK somehow.