Simply destroying row heights cache

There is no reason to neither check that the pointer is non-null nor to
call Clear() on it before deleting it, so don't do it.

src/generic/datavgen.cpp

@@ -1992,11 +1992,7 @@ wxDataViewMainWindow::~wxDataViewMainWindow()
 {
     DestroyTree();
     delete m_renameTimer;
-    if (m_rowHeightCache != NULL)
+    delete m_rowHeightCache;
-    {
-        m_rowHeightCache->Clear();
-        delete m_rowHeightCache;
-    }
 }