Finish removing the user agent from sent emails.

Summary:
Commit 6296818e9c7003bec99 (https://bugs.kde.org/show_bug.cgi?id=359964)
only did it partially, the unittest messagefactorytest was still failing
due to the useragent being present in many cases.

This fixes many test failures in that test, there's still one though:
testCreateReplyToAllWithUseSenderAndIdentityInCC shows a
"X-KMail-Identity: 2109233399" header and a wrong From field.

Test Plan: messagefactorytest

Reviewers: mlaurent, dvratil

Reviewed By: mlaurent, dvratil

Subscribers: KDE PIM

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D4809