DICOM/Widgets: Fix "unused-but-set-variable" warning
DICOM/Widgets: Fix "unused-but-set-variable" warning

This commit fixes the following warning:

/path/to/CTK/Libs/DICOM/Widgets/ctkDICOMThumbnailListWidget.cpp: In member function ‘void ctkDICOMThumbnailListWidgetPrivate::addSeriesThumbnails(const QModelIndex&)’:
/path/to/CTK/Libs/DICOM/Widgets/ctkDICOMThumbnailListWidget.cpp:149:15: warning: variable ‘studyIndex’ set but not used [-Wunused-but-set-variable]
   QModelIndex studyIndex = index.parent();
               ^

commontk/CTK,SINTEFMedtek/CTK,SINTEFMedtek/CTK,SINTEFMedtek/CTK,commontk/CTK,CJGoch/CTK,SINTEFMedtek/CTK,jcfr/CTK,CJGoch/CTK,CJGoch/CTK,jcfr/CTK,SINTEFMedtek/CTK,CJGoch/CTK,jcfr/CTK,commontk/CTK,jcfr/CTK,CJGoch/CTK,commontk/CTK,jcfr/CTK