Remove warnings about ignoring return value of removeHeader()

Summary:
Most uses of Content::RemoveHeader() do not check its return value;
they want to remove the header if it is present, but don't care if it
isn't. This change silences the unnecessary compilation warnings.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D24484