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.h

@@ -82,8 +82,6 @@ private:
     QVariantMap toMap() const override;
     BuildType buildType() const override;
 
-    QString disabledReason() const override;
-
     ProjectExplorer::NamedWidget *createConfigWidget() override;
 
     bool fromMap(const QVariantMap &map) override;