[Indic] Don't split U+0AC9

Althought IndicMatraCategory.txt classifies it as Top_And_Right matra,
it does not have Unicode decomposition, and Uniscribe does not do
anything special about it either.

Gujarati failures down from 0.672% to 0.0130966%.

src/hb-unicode.cc

@@ -269,7 +269,7 @@ hb_unicode_decompose (hb_unicode_funcs_t *ufuncs,
 {
   /* XXX FIXME, move these to complex shapers and propagage to normalizer.*/
   switch (ab) {
-    case 0x0AC9  : *a = 0x0AC5; *b= 0x0ABE; return true;
+    case 0x0AC9  : return false;
 
     case 0x0931  : return false;
     case 0x0B94  : return false;