[kerx] No-op

Tested that Format0 works with Kannada MN font:

$ make -j5 lib -s && HB_OPTIONS=aat ./hb-shape Kannada\ MN.ttc -u 0C95,0CC2
[kn_ka=0+1000|kn_matra_uu=0@-30,0+1345]

$ make -j5 lib -s && HB_OPTIONS=aat ./hb-shape Kannada\ MN.ttc -u 0C95,0CC2 --features=-kern
[kn_ka=0+1030|kn_matra_uu=0+1375]

Note that GPOS does the same with 'dist' feature, and applies the whole difference to the
same glyph:

$ make -j5 lib -s && ./hb-shape Kannada\ MN.ttc -u 0C95,0CC2
[kn_ka=0+970|kn_matra_uu=0+1375]

$ make -j5 lib -s && ./hb-shape Kannada\ MN.ttc -u 0C95,0CC2 --features=-dist
[kn_ka=0+1030|kn_matra_uu=0+1375]

src/hb-aat-layout-kerx-table.hh

@@ -66,9 +66,7 @@ struct KerxSubTableFormat0
   {
     hb_glyph_pair_t pair = {left, right};
     int i = pairs.bsearch (pair);
-    if (i == -1)
-      return 0;
-    return pairs[i].get_kerning ();
+    return i == -1 ? 0 : pairs[i].get_kerning ();
   }
 
   inline bool apply (hb_aat_apply_context_t *c) const