Remove Environment cleanup

Some managers may use the environment in the destructors. Setting them to
nullptr may lead to nullptr dereference when the object is still alive and can
be accessible. But after object is destructed it's UB anyway to dereference
nullptr or a dangling pointer.

apps/openmw/engine.cpp

@@ -468,8 +468,6 @@ OMW::Engine::~Engine()
 
     mStereoManager = nullptr;
 
-    mEnvironment.cleanup();
-
     mMechanicsManager = nullptr;
     mDialogueManager = nullptr;
     mJournal = nullptr;

apps/openmw/mwbase/environment.cpp

@@ -28,21 +28,6 @@ MWBase::Environment::~Environment()
     sThis = nullptr;
 }
 
-void MWBase::Environment::cleanup()
-{
-    mMechanicsManager = nullptr;
-    mDialogueManager = nullptr;
-    mJournal = nullptr;
-    mScriptManager = nullptr;
-    mWindowManager = nullptr;
-    mWorld = nullptr;
-    mSoundManager = nullptr;
-    mInputManager = nullptr;
-    mStateManager = nullptr;
-    mLuaManager = nullptr;
-    mResourceSystem = nullptr;
-}
-
 void MWBase::Environment::reportStats(unsigned int frameNumber, osg::Stats& stats) const
 {
     mMechanicsManager->reportStats(frameNumber, stats);

apps/openmw/mwbase/environment.hpp

@@ -112,8 +112,6 @@ namespace MWBase
 
             void setFrameDuration(float value) { mFrameDuration = value; }
 
-            void cleanup();
-
             /// Return instance of this class.
             static const Environment& get()
             {