Revert "#327 - Add a new extension manager for structured content fields"

This reverts commit c61335e429670f26b0f47070ea86d6652a02dea2.
Merge branch 'BroadleafCommerce-3.0.x' into develop
the 3.0 line cannot take advantage of them as they were not originally
placed in 'common'.

This reverts commit da98c51911c40296d2785f26e37b08f82c354192.