[gobject] Remove hb_language_t workarounds for g-i shortcomings

Using latest gobject-introspection, I don't seem to be having this
problem anymore:

  https://bugzilla.gnome.org/show_bug.cgi?id=707656

Removing that kludge makes language_t behave more like the way I expect it
in Python.

Also fixes:
https://github.com/behdad/harfbuzz/issues/91

src/hb-gobject-structs.cc

@@ -78,4 +78,3 @@ HB_DEFINE_VALUE_TYPE (glyph_info)
 HB_DEFINE_VALUE_TYPE (glyph_position)
 HB_DEFINE_VALUE_TYPE (segment_properties)
 HB_DEFINE_VALUE_TYPE (user_data_key)
-HB_DEFINE_VALUE_TYPE (language)

src/hb-gobject-structs.h

@@ -81,14 +81,6 @@ GType hb_gobject_segment_properties_get_type (void);
 GType hb_gobject_user_data_key_get_type (void);
 #define HB_GOBJECT_TYPE_USER_DATA_KEY (hb_gobject_user_data_key_get_type ())
 
-/* Currently gobject-introspection doesn't understand that hb_language_t
- * can be passed by-value.  As such we box it up.  May remove in the
- * future.
- *
- *   https://bugzilla.gnome.org/show_bug.cgi?id=707656
- */
-GType hb_gobject_language_get_type (void);
-#define HB_GOBJECT_TYPE_LANGUAGE (hb_gobject_language_get_type ())
 
 HB_END_DECLS