[introspection] Disable constructors for now

Since our types are not associated with their methods, marking
constructors makes them inaccessible from bindings.  Undo for now.

src/hb-unicode.cc

diff --git a/src/hb-unicode.cc b/src/hb-unicode.cc
index 807d5c1..5b44913 100644
--- a/src/hb-unicode.cc
+++ b/src/hb-unicode.cc
@@ -151,7 +151,7 @@
 #endif
 
 /**
- * hb_unicode_funcs_create: (constructor)
+ * hb_unicode_funcs_create: (Xconstructor)
  * @parent: (allow-none):
  *
  *