Reintegrate textproc/clucene-qt5 into devel/qt5-help

As devel/qt5-help is the only consumer of textproc/clucene-qt5 and the latter
it is part of the same distfile we can simplify it, and get rid of clucene-qt5.

This was spotted by Marie Loise Nolden <nolden@kde.org> -- thanks :)

Reviewed by: rakuco
Approved by: rakuco (mentor)
Sponsored by: https://reviews.freebsd.org/D8638