Remove hook_entity_bundle_info()'s need for 'real path'

#1946404: Change notice: Convert forms in field_ui.admin.inc to the new form interface removed 90% of the Field UI 'admin' section of hook_entity_bundle_info(), leaving only 'real path', which is used almost exclusively by Field UI confirmation forms and the "Field List" admin reports page.

At this point it can be automatically derived, no more need for module developers to specify it manually.