This method was confusing and dangerous as it didn't actually initialize the
wxColour object because it didn't set its m_cgColour and so attempting to use
wxColour "initialized" using this method resulted in a crash whenever its
GetPixel(), GetCGColor() or CreateCGColor() methods were called.
Just merge this function with InitCGColorRef() which is the only remaining
place where it's used after rewriting wxColour(NSColor) ctor.
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