CMake: Cache cmake configuration only once

This fixes saves memory and fixes some update issues with the cmake
configuration, where the three layers of caches used to interfere
with each other.

Task-number: QTCREATORBUG-17360
Change-Id: I5564bbe46ca8de6b38dd710100bfc18fad98eac5
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildconfiguration.h

@@ -118,8 +118,6 @@ private:
     QString m_error;
     QString m_warning;
 
-    mutable QList<CMakeConfigItem> m_completeConfigurationCache;
-
     std::unique_ptr<BuildDirManager> m_buildDirManager;
 
     friend class CMakeBuildSettingsWidget;