NEXUS-2834: rebuild md on ui actions

It was kicking in for every file deletion made from anywhere,
clobbing tasks like snap remover that were removing items
in a batch (md was rebuilt for every file removed, even if whole
GAV was about to be deleted). Moreover, tasks like
snap remover DO rebuild MD when done, but in optmized
fashion, see ParentOMatic use et al.

Related issues:
https://issues.sonatype.org/browse/NEXUS-2834
https://issues.sonatype.org/browse/NEXUS-5773

CI:
TBD