Build qhelpgenerator regardless of the assistant feature

qhelpgenerator doesn't depend on Assistant, but it was only added
when FEATURE_assistant was enabled. Add the help library, qhelpgenerator
and the plugins unconditionally, and keep only assistant (and its
qlitehtml dependency) behind the feature.

Pick-to: 6.12
Fixes: QTBUG-136345
Change-Id: I8a5051296a54c26545576d503cfe0f994ef02c02
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>