Fix reshare permission change to not impair other deletion code

A recent change that prevents reshare permission changes to delete group
share children had the side-effect of also preventing group share
children deletion when it needed to be done.

This fix adds an extra flag to isolate the "reshare permission change"
deletion case and keep the other ones as they were before, not only to
fix the regression but also fix other potential regressions in code that
uses this method.

Also updated the comment because now Helper::delete() is no longer
limited to reshares but also applies to group share children.