Remove KMime::HeaderBase::clear()
This has meanwhile no external users anymore, and it's not ideal API as it puts headers in a kind of existing but invalid state for most of them.
See also https://phabricator.kde.org/T615.
This has meanwhile no external users anymore, and it's not ideal API as it puts headers in a kind of existing but invalid state for most of them.
See also https://phabricator.kde.org/T615.