Remove wxDIB::m_hasAlpha from wxMSW.

This field can't be set reliably as we don't know if LoadImage() Windows
function loaded an 0RGB or an RGB bitmap so remove it completely to avoid the
risk of using it wrongly.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65899 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/dib.h

@@ -200,11 +200,6 @@ private:
     // the case
     bool m_ownsHandle;
 
-    // if true, we have alpha, if false we don't (note that we can still have
-    // m_depth == 32 but the last component is then simply padding and not
-    // alpha)
-    bool m_hasAlpha;
-
 
     // DIBs can't be copied
     wxDIB(const wxDIB&);
@@ -220,7 +215,6 @@ void wxDIB::Init()
 {
     m_handle = 0;
     m_ownsHandle = true;
-    m_hasAlpha = false;
 
     m_data = NULL;