CMake: Remove disabledReason

The current implementation does not help the user and is not in sync with isEnabled(),
so it is not helpful to have.

Change-Id: Iddde1ac3da24d50e8a16d2bd4c6aa56eef34ef50
Reviewed-by: hjk <hjk@qt.io>

src/plugins/cmakeprojectmanager/cmakebuildconfiguration.cpp

@@ -171,12 +171,6 @@ CMakeBuildConfiguration::~CMakeBuildConfiguration()
     delete m_buildSystem;
 }
 
-
-QString CMakeBuildConfiguration::disabledReason() const
-{
-    return error();
-}
-
 QVariantMap CMakeBuildConfiguration::toMap() const
 {
     QVariantMap map(ProjectExplorer::BuildConfiguration::toMap());