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.

interface/wx/bitmap.h

@@ -816,21 +816,6 @@ public:
         When creating a new bitmap, CreateScaled() can be used to specify the
         correct scale factor from the beginning.
 
-        Note that this method exists in all ports, but simply does nothing in
-        those of them that don't use logical pixel scaling. The preprocessor
-        symbol @c wxHAS_BITMAP_SCALE_FACTOR can be tested to determine whether
-        the scale factor is really supported, e.g.
-
-        @code
-            bitmap.SetScaleFactor(2);
-
-            // In the other ports scale factor is always 1, so the assert would
-            // fail there.
-            #ifdef wxHAS_BITMAP_SCALE_FACTOR
-                wxASSERT( bitmap.GetScaleFactor() == 2 );
-            #endif
-        @endcode
-
         @since 3.1.6
      */
     virtual void SetScaleFactor(double scale);