Now wxMSW also stores the scale factor, even if it doesn't use it in its
GetScaledXXX(), so it doesn't seem useful to have this symbol for
distinguishing the platforms with and without bitmap scale factor
support, when we can just use wxHAS_DPI_INDEPENDENT_PIXELS instead in
the only place where this was used.
And as this symbol was added quite recently, in 2c1f4c002d (Add
wxBitmap::SetScaleFactor(), 2021-10-23), we can hopefully just remove it
without breaking any existing code, if we do it right now.
03cf1f4359
