Don't convert any non-monochrome wxBitmap to ARGB format

While selecting wxBitmap with colour depth > 1 bpp into wxMemoryDC there is
no need to convert its format to ARGB regardless of the current
interpretation of the alpha channel values in the bitmap. If original
bitmap was marked as a RGB one (with alpha channel being ignored), it
should retain this format to avoid confusions.

Closes #16309.

src/osx/core/dcmemory.cpp

@@ -72,8 +72,6 @@ void wxMemoryDCImpl::DoSelect( const wxBitmap& bitmap )
     m_selected = bitmap;
     if (m_selected.IsOk())
     {
-        if ( m_selected.GetDepth() != 1 )
-            m_selected.UseAlpha() ;
         m_selected.BeginRawAccess() ;
         m_width = bitmap.GetScaledWidth();
         m_height = bitmap.GetScaledHeight();