Drop unused functions from storage

Summary:
These functions are not used, and I see no sign of their
previous callers.
In my opinion, they do not make much sense : these operations
belong to a higher level that would pass the data around using
the same API as when fetching/viewing articles.

Reviewers: knauss

Reviewed By: knauss

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D12450