Remove wxHAS_BITMAP_SCALE_FACTOR

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.

docs/doxygen/mainpages/const_cpp.h

@@ -167,9 +167,6 @@ Currently the following symbols exist:
     have an efficient (CPU-specific) implementation. Notice that the functions
     themselves are always available but can be prohibitively slow to use when
     implemented in a generic way, using a critical section.}
-@itemdef{wxHAS_BITMAP_SCALE_FACTOR, Defined in @c wx/bitmap.h if bitmaps
-    actually use scale factor under the current platform, see
-    wxBitmap::SetScaleFactor().}
 @itemdef{wxHAS_BITMAPTOGGLEBUTTON, Defined in @c wx/tglbtn.h if
     wxBitmapToggleButton class is available in addition to wxToggleButton.}
 @itemdef{wxHAS_CONFIG_TEMPLATE_RW, Defined if the currently used compiler