COMP: Fix unused variable warning in qSlicerCoreApplication

This commit fixes the following warning occurring when Slicer is build
with extension support is disabled.

/path/to/Slicer/Base/QTCore/qSlicerCoreApplication.cxx: In member function ‘QString qSlicerCoreApplication::defaultExtensionsInstallPath() const’:
/path/to/Slicer/Base/QTCore/qSlicerCoreApplication.cxx:1159:14: warning: unused variable ‘appSettings’ [-Wunused-variable]
QSettings* appSettings = this->userSettings();

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23601 3bd1e089-480b-0410-8dfb-8563597acbee