fix inline migrator for AT types and add option to remove data in old field

adding a test for this is a lot of work since we'd need a separate contenttype using an imagefield and register a schemaextender using a blobimagefield for it. (similar to the example provided in the [upgrade guide](https://plone.org/documentation/manual/upgrade-guide/version/upgrading-plone-3-x-to-4.0/updating-add-on-products-for-plone-4.0/use-plone.app.blob-based-blob-storage))

see https://github.com/plone/plone.app.blob/commit/f57656ee2c5e483fd02db0d2ac256d6e98d9e609 for initial discussion.