Prefer use of vips_interpretation_max_alpha()

This also ensures we handle scRGB correctly, see:
e9c5a31552

src/common.h

@@ -357,12 +357,6 @@ namespace sharp {
   */
   bool Is16Bit(VipsInterpretation const interpretation);
 
-  /*
-    Return the image alpha maximum. Useful for combining alpha bands. scRGB
-    images are 0 - 1 for image data, but the alpha is 0 - 255.
-  */
-  double MaximumImageAlpha(VipsInterpretation const interpretation);
-
   /*
     Convert RGBA value to another colourspace
   */