[Kuit] Port QRegExp to QRegularExpression

Summary:
Port salvageMarkup() to QRegularExpression.

Remove one redundant QRegExp #include.

With this commit all of KI18n has been ported to QRegularExpression.

Test Plan: make && ctest

Reviewers: Frameworks, ilic, dfaure, mlaurent, aacid

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26412