CMakePM: Remove grouping of cmake cache variables

Having groups made from the first underscore is not necessarily
helpful.

In some cases it creates empty groups, which can confuse users.

We have filter and search functionality in the list, which can
help with finding the right items.

Fixes: QTCREATORBUG-26218
Change-Id: I751a59b49184ea57f55d3832ef4edc8a3c33e0da
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildconfiguration.cpp

@@ -271,6 +271,7 @@ CMakeBuildSettingsWidget::CMakeBuildSettingsWidget(CMakeBuildConfiguration *bc)
     m_configView->setFrameShape(QFrame::NoFrame);
     m_configView->setItemDelegate(new ConfigModelItemDelegate(m_buildConfiguration->project()->projectDirectory(),
                                                               m_configView));
+    m_configView->setRootIsDecorated(false);
     QFrame *findWrapper = Core::ItemViewFind::createSearchableWrapper(m_configView, Core::ItemViewFind::LightColored);
     findWrapper->setFrameStyle(QFrame::StyledPanel);