Revert "Treat HAMZA ABOVE similar to SHADD for sorting purposes"

This reverts commit 5a330575.

Proper fix coming soon.

src/hb-unicode-private.hh

@@ -105,10 +105,6 @@ HB_UNICODE_FUNCS_IMPLEMENT_CALLBACKS_SIMPLE
   inline unsigned int
   modified_combining_class (hb_codepoint_t unicode)
   {
-    /* XXX This hack belongs to the Arabic shaper:
-     * Put HAMZA ABOVE in the same class as SHADDA. */
-    if (unlikely (unicode == 0x0654u)) unicode = 0x0651u;
-
     /* XXX This hack belongs to the Myanmar shaper. */
     if (unlikely (unicode == 0x1037u)) unicode = 0x103Au;