CMake: Remove unnecessary members from CMakeTargets

Change-Id: I6428496f4eab0d42f44525477c716f74714f8824
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/cmakeprojectmanager/cmakeproject.cpp

@@ -642,7 +642,6 @@ void CMakeBuildTarget::clear()
 {
     executable.clear();
     makeCommand.clear();
-    makeCleanCommand.clear();
     workingDirectory.clear();
     sourceDirectory.clear();
     title.clear();