[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-font.cc

diff --git a/src/hb-font.cc b/src/hb-font.cc
index f02afa2..42e0cee 100644
--- a/src/hb-font.cc
+++ b/src/hb-font.cc
@@ -231,7 +231,7 @@
 
 
 /**
- * hb_font_funcs_create: (constructor)
+ * hb_font_funcs_create: (Xconstructor)
  *
  * 
  *
@@ -839,7 +839,7 @@
  */
 
 /**
- * hb_font_create: (constructor)
+ * hb_font_create: (Xconstructor)
  * @face: a face.
  *
  *