Remove a useless assert from wxDataViewTreeNode::GetChildNodes()

Checking that a pointer is non-null before dereferencing it is perfectly
useless: the code will crash anyhow, so assert doesn't help with
debugging it in debug builds nor with preventing the crash in release.

src/generic/datavgen.cpp

@@ -496,7 +496,6 @@ public:
 
     const wxDataViewTreeNodes& GetChildNodes() const
     {
-        wxASSERT( m_branchData != NULL );
         return m_branchData->children;
     }