Remove premature wxAuiNotebook::m_tabCtrlHeight initialization

FromDIP() shouldn't be used before the window is created and
m_tabCtrlHeight was already initialized correctly in InitNotebook(),
when it can be used, so simply remove this premature and redundant
initialization.

src/aui/auibook.cpp

@@ -1713,7 +1713,6 @@ void wxAuiNotebook::Init()
     m_curPage = -1;
     m_tabIdCounter = wxAuiBaseTabCtrlId;
     m_dummyWnd = NULL;
-    m_tabCtrlHeight = FromDIP(20);
     m_requestedBmpSize = wxDefaultSize;
     m_requestedTabCtrlHeight = -1;
 }