EZP-21324: Fixed regression when deleting draft

Also changed removeAlias code. Should now check correctly:
- only remove ezimagealias entries when they're not used anymore
  (handles duplicates if any)
- only removed files when not used anymore.

This means that we accept that when a copy of a content with an
image is made, we accept that archived versions in the copy will
reference the same file than archive versions in the copy source.