Flush ContentState during FeatureType DELETE (GEOS-6616)

First fix for better management of attribute cache information (GEOS-6616)

For details see: https://jira.codehaus.org/browse/GEOS-6616
In case it's not ContentDataStore, we should drop the store from the resource pool cache, as building the feature types is expsensive in many cases, so performance sensitive stores will be coded to cache it.