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