NEXUS-7628: On delete of collection purge NFC for children

deleteItem method originally operated on file items only. Later, it was "extended" to cope with collections too (by adding that Walker thingy to perform notifications recursively), but NFC was never handled properly if collection was being deleted.

Issue
https://issues.sonatype.org/browse/NEXUS-7628

CI
http://bamboo.s/browse/NX-OSSF368