Remove some warnings about unused return values.

Summary:
RichTextComposerSignatures::replaceSignature(),
AttachmentModel::addAttachment(), and Item::appendChildItem() are called
for their side effects, and their returned values are not error indicators,
so there's no need to check them.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: mlaurent, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D24709