Doc: G4 multithreading OFF is not required anymore.

(cherry picked from commit b4b3126)

Author: simonspa

doc/usermanual/chapters/installation.tex

@@ -33,7 +33,6 @@ \subsection{Prerequisites}
 A useful set of CMake flags to build a functional Geant4 package would be:
 \begin{verbatim}
 -DGEANT4_INSTALL_DATA=ON
--DGEANT4_BUILD_MULTITHREADED=OFF
 -DGEANT4_USE_GDML=ON
 -DGEANT4_USE_QT=ON
 -DGEANT4_USE_XM=ON