[graphite] Stop creating unused gr_face

It is unused after previous commit, hb_graphite2_font_get_gr_font()
makes no sense now so deprecating it.

src/hb-graphite2.h

diff --git a/src/hb-graphite2.h b/src/hb-graphite2.h
index 122c3e4..82b1e64 100644
--- a/src/hb-graphite2.h
+++ b/src/hb-graphite2.h
@@ -39,9 +39,13 @@
 HB_EXTERN gr_face *
 hb_graphite2_face_get_gr_face (hb_face_t *face);
 
+#ifndef HB_DISABLE_DEPRECATED
+
 HB_EXTERN gr_font *
 hb_graphite2_font_get_gr_font (hb_font_t *font);
 
+#endif
+
 
 HB_END_DECLS