[subset] Disable GSUB/GPOS subsetting for now

So I can get a release out.  I haven't debugged those yet, and they
are producing bad tables.

src/hb-subset.cc

@@ -179,10 +179,10 @@ _subset_table (hb_subset_plan_t *plan,
       break;
 
     case HB_OT_TAG_GSUB:
-      result = _subset2<const OT::GSUB> (plan);
+      //result = _subset2<const OT::GSUB> (plan);
       break;
     case HB_OT_TAG_GPOS:
-      result = _subset2<const OT::GPOS> (plan);
+      //result = _subset2<const OT::GPOS> (plan);
       break;
 
     default: